Just taking a quick look at competitors, it looks like 200 is a pretty common limit in the industry, but the difference is that most of the competitors allow you to have more than one controller/hub on an account, so you just add more if you need more devices.
For example, control 4 has several different controllers going up to a maximum of 200 each, but at least back in 2017 a typical “whole house install“ had 4 to 6 controllers.
Lutron’s high end system Homework’s, limits you to 100 devices per controller, but you can have 32 controllers on an account. Also, there’s a separate limit of up to 10,000 endpoints, which they called “zones.”
HomeKit limits you to 150 Devices per Bridge, but doesn’t limit the number of bridges, and allows a total number of 1000 devices per account. (Some people hit a limit earlier than that because all the device names get written to a single iCloud record and there’s a maximum memory size on that, but the workaround is to give the devices shorter names.)
The original Vera used the zwave hard limit of 232 devices per hub, but again it was easy to add additional hubs to the account.
Zigbee coordinators don’t tend to have a hard limit, but will run out of memory to process everything, and that is typically around 200 devices although it varies.
Insteon allows for around 1,000 “links,” which seem to include both scenes and endpoints, with a hard limit of 100 devices. They say that the average American home has 80 switches and outlets in it. Note that the issue here is sensors, it’s really easy to add dozens of sensors to a smart home Once you go for full automation. These days it looks like people get around the 100 device per hub limitation by either using HomeKit or ISY with multiple Insteon controllers.
I should add that they are also quite a few low cost security-only systems which have a limit of around 150 devices, but then they are not doing lights. It’s when you allow for both lighting control and sensors, as SmartThings does, that your power users will start to go above 200 devices.
I would also add that smartthings bring some of this on themselves with their parent/child Architecture. Most other systems don’t list these as different devices: they are multiple endpoints on one device. That’s where the Lutron “zones“ and the Insteon “links“ come in, and why these companies have much higher maximums than just the number of devices.
The main exception I could find with a quick look was Homeseer. Homeseer allows up to 10,000 devices. There’s a default setting in the app which is limited to displaying about 500, but that’s just for efficiency: you can change it in the parameter settings.
So my suggestion would be to do what Homeseer does: set a default lower display limit so the app will load quickly for most people but allow a parameter change for those who have bigger installations. But either make multiple hub use easy or increase the device count in order to compete effectively with both Homekit and Homeseer.
I would suggest that the current limit is based on systems which are primarily for lighting, and doesn’t take into account the issue of sensors and plug-ins, including virtual switches. Both are device categories which can pretty easily take a power user over a limit of 200 devices. Also, smart bulbs have upped the potential Device count as well.
Just an observation… 
