In a post by SmartThings Software Engineer @jody.albritton on that same thread you linked to:
Custom smartapps and devices are not going away. Check out the new developer portal (very beta)
I’ve poked around the new developer website and looked at documentation. It appears that they are moving away from runtime-compiled groovy on ST’s servers as the specific SmartApp / Device Handler platform, and over to Amazon Web Services (AWS) Lambda functions / web services applications with a RESTful API interface.
Officially supported languages on AWS Lambda are Node.js, Python, Java and C# through .NET Core. I am not a programmer by any stretch of the imagination, but since Groovy is a language for the Java platform and compiled into Java Virtual Machine bytecode, I’d have to guess that moving current SmartApps and Device Handlers over will be a matter of porting with minor modifications rather than complete re-writes. It really depends on how much of the currently available APIs and functions are moved over to the new platform. Also, SmartThings’ Groovy code runs in a restricted sandboxed environment, and I haven’t read anything in the new developer documentation that explains whether that will be the case with code running on AWS Lambda.
One thing for sure is that cost structuring will change. AWS Lambda does have a free tier usage level, but when you go past limits, you are charged. As to who gets charged, it depends on whether the code is hosted on SmartThings / Samsung’s AWS-L account, the SmartApp / Device developer’s account, or the end-user’s account (assuming the new ST infrastructure will allow for end-user’s to host custom code on AWS-L).
Also, I read things like this page about Rate Limits on the new developer documentation site, and I really want someone to explain how this will affect me as an end-user. Will I have to worry about how many devices I can have connected to my hub or interfacing with a SmartApp?