One Time Investment Systems
I understand what you’re saying, and if you want to get home automation with a one time investment you absolutely can: but it has to be something that works entirely locally and you have to be able to defer or deny updates from each manufacturer of every part of your system.
Indigo Domotics, for example, is software that runs on a Mac and works with Z wave or Insteon or x10 devices when you add a memory stick. Once you have that, you can run it forever. They also sell “plug-ins“ which offer compatibility with a number of other cloud-based devices, but then you have the same vulnerability as you have with any cloud.
I have friends who have run lighting setups on indigo for 10 years without ever making any changes to it. and without buying any new devices. But they run it without including Smart Locks (indigo only added support for locks a couple of years ago), video anything, or voice assistants. Basically they are frozen in time from the time they bought the system.
There are some other similar systems available. Anything that worked on Monday will continue to work on Tuesday until the hardware physically wears out. but you may not be able to support new features that come on the market and you have to stay away from the cloud altogether. 
Basically if someone else can make changes to your system without your knowledge, which is true of almost all cloud systems, then you can’t protect your sunk costs.
If you get notice but you can’t deny updates, again, you can’t protect your sunk costs.
Home Automation as a Service (HASS)
So I made the decision that I wanted the rapidly evolving new features in Home automation as it stands today, but that meant I had to assume a fairly short obsolescence curve for whatever I was buying. I settled on three years as a rule of thumb, and it’s worked quite well.
As far as budgets, I’m pretty careful in this regard. In the beginning I had a budget of $500 per room and a total of $5000 for the house, but my first phase budget was only $1500 while I tried things out, so I expected I might have to replace all of those items after 18 months. I’ve detailed my expenditures during that phase elsewhere.
However, during that time the echo was introduced and I realized new features were being added too rapidly for me to stick to my original plan.
So I looked at what I could afford and what I wanted to accomplish and decided that I would be OK with a budget of $125/month for Home automation hardware and services, which is very similar to my cell phone bill. The difference is that I spend much less on Services, but I do have some monthly bills there.
Anyway, obviously that’s a lot of money, but I’m OK with spending that much each month to meet the specific use cases I want covered: voice control of my television, A hands-free smart lock, voice control of my HVAC including the window air conditioner, at least one voice operable light in each of the main rooms, voice operable lights in my bedroom, and pathway lights in several areas. I don’t run security off of this system, that’s a separate budget.
Someone else would have different critical use cases, for example I don’t need garage door control, but these are mine.
And I saw that I could meet these for my target monthly budget and in fact have some room for additional devices like a video doorbell.
Is $125 a month a lot for this? It would be for some people. For others it would be very little. And for quite a few, including me, it would be an acceptable expense for my critical use cases.
It’s basically my original budget spread out over three years. So it’s more than I initially thought I would spend, because I think I had some idea in my head that the original budget would be more like plumbing or light fixtures and last 10 or 20 years. But it’s an acceptable way of looking at it for me and an acceptable amount of money for the services I’m getting.
What I Learned About What I Didn’t Know
the echo experience taught me that I don’t know enough now to predict what features I will want in the future. Smartthings experience taught me that I am literally not safe if I have my critical use cases on a cloud that can change without warning. And my experience with Apple HomeKit taught me that when the technology is changing this quickly, sometimes prices for hardware come down quite dramatically as well.
So I didn’t want to be locked into choices I had made five years ago, but I also wanted significant control over when and if changes will be made in the features that I use for critical use cases.
My current system isn’t perfect in this regard. I do use echo a lot and Amazon can change those features without prior notice, although so far every change they’ve made has been an improvement from my perspective.
I moved my critical use cases to HomeKit in 2016, which means I get to decide when and if any updates are applied and nothing mysteriously stops working because of a platform change. But it also means I am continually being offered new features and new devices should I choose to take advantage of them. 
Summary
Different approaches will work for different people. If you want a one time cost and no changes, there are home automation systems that can offer that. (But cloud-based systems will not fall into that category).
If you want to look at home automation as a service, you can come up with a pretty accurate monthly budget, but my personal experience is that you should expect a three-year replacement cycle.
If you want a cloud-based system that protects your sunk costs, you’ll have to read the fine print very very carefully and opt for something that has a continuous service guarantee. Which definitely isn’t smartthings. 
From the smartthings terms of use:
Samsung Account
Will SmartThings ever change the Services?
.
We’re always trying to improve the Services, so they may change over time. We may suspend or discontinue any part of the Services, or we may introduce new features or impose limits on certain features or restrict access to parts or all of the Services.
.
In some cases, the changes we make to the Services may cause older hardware devices, third party services, software configurations or setups to no longer work with the Services, and you may be required to upgrade or change these devices, services, configurations or setups in order to continue using the Services.
.
We’ll try to give you notice when we make a material change to the Services that would adversely affect you, but this isn’t always practical.
.
Similarly, we reserve the right to remove any Content (including any SmartApps or device or external service connections provided by SmartThings or by third parties) from the Services at any time, for any reason (including, but not limited to, if someone alleges you contributed that Content in violation of these Terms), in our sole discretion, and without notice.
Every smartthings customer agreed to that when they created their account. I know not everybody reads the fine print, and I know it’s frustrating when you realize that what you have isn’t what you thought you had, but at least you’ll know that there are alternatives if you want to look for them. 