Not true.Ren'Py. If you change, add, remove variables...you break saves. (there's likely more to it, but simplest explanation)
Most Devs try to avoid this with Ren'Py...others just charge ahead...saves be damned.
You can declare variables as "default" in Renpy. These variables work independently of your save and are read every time the game restarts. If you don't radically change the structure of the game, you can keep the saves working from the tech demo to the final version.
The point is: Renpy is a project that has existed for more than a decade and its evolution has been influenced by thousands of amateur and professional devs. The issue of saves working is not only a convenience for the players, it is an essential tool for the developer. Without saves that work between versions, you basically can't test your game.
When you as a developer decide to "improve" an engine to make your game possible, the first thing you need to do is: get to know the engine deeply. Otherwise, your improvements will destroy more than a decade of work and solutions that were found in 2005 and that were appreciated by countless devs, are suddenly undone and you start going through bad times...