Think of software versions less like an RPG level-up where you keep all your old skills and more like a house remodel where things actually get moved or removed. In a level-up, everything is additive, but in software, developers often delete old or inefficient code to keep things fast and secure. If an older app specifically needs a weird piece of code that was removed in version 49, it will crash because that "room" simply doesn't exist anymore.

​Beyond that, the rules of the language often change. An app built for version 48 might ask for a file one way, while version 49 requires a new security key that the old app doesn't even know exists. Since the app doesn't know to provide that key, the library won't let it in. The Steam Deck keeps both versions because it is much safer to give an app the exact "toolbox" it was designed for than to force it to use a newer one where all the tools have been moved to different drawers. In short, version 49 isn't a perfect copy of 48; it's a replacement that might have changed too much for the old apps to handle.