Lumping DCC in with DCS and TMCC is a fools errand. DCC is an open system. External parties can bring their stuff to the party and nicely with others. Bluetooth and WiFi, should be this way, but are only over the short term because of evolving standards and security issues.
The app stores are a whole other ball of wax. They are a closed system that Lionel and MTH don't control.
Bluetooth and Wifi are constantly evolving standards, that is true, but the thing is that for example WiFi 6 maintains backwards support for all the prior wifi standards, like n, ac, etc . Bluetooth does the same thing, it extends but if you have a bluetooth device connecting to one with a more modern version of bluetooth, it should work and usually does.
Yes, you can have problems with Apps and IOS/Android OS upgrades. While like any OS they try and maintain backwards capability to allow older apps to work, there will come a time when an older app may fail. The other issue is there as well, there does come a time when an older device won't be able to run a new version of the app because it has an older ops system and can't upgrade. I realize some people keep old hardware and such and are happy with it, but you also can't expect it to run newer applications forever, it is the nature of how things move on. So someday, yes, you might have to buy a new Ipad or Iphone or android device, expecting something from 10 years ago to run something from today is kind of ridiculous IMO.
So a couple of points:
1)If we get to the point that the Lionel or MTH apps no longer work with the latest generation of OS's, then basically that means they have stopped supporting the product, which would be big problem because most people end up updating their iphones and ipad and android systems, either the OS or to a new device. Hopefully if they ever did get to that point, they might make it open source so others can update it to work or add new features, otherwise it will basically end up a brick.
I doubt this, for a lot of reasons, it seems like if and when the app no longer works, they will issue updates.
2)You have the same problem with proprietary systems using controllers, you are even more at their mercy with that. Sure, your 1997 TMCC command base and controller might work great, but for example, you are limited only to the TMCC command set. If the controller breaks, good luck trying to fix it....the key thing here being you lost support for what you have, and if it has a problem, like an App if you upgrade your os, it means you can be out of luck.
The nice part about the software approach is that it is relatively easy to fix an app to run under a new OS , it is a lot cheaper and easier than maintaining a physical remote, that if a part is defective they have to recall the unit and fix it, if it is a manufacturing process issue, even a bigger headache. Also very costly, something like keeping their app compatible with newer os's can be farmed out pretty easily (new features they may want to keep in house). And if let's say MTH or lionel ever 'goes out', they likely would release the technology to the public domain or sell it to someone who will maintain it IMO.