I'm sorry, but that doesn't make any sense.I'm sure it is. But it's still not Renpy. I vaguely skimmed the documentation and it's already implementing it's own UI.
The point of Renpy is that you don't need more. Everything you'd want is already in there, you're talking about creating UI, but these are VNs, players don't give a fuck about your intricately designed UI. You talk about more features, but more features are never free, they add loading time, they add a maintenance cost, and they add complexity. I'll remind you that you're not making games so that you can admire how well you made it, but so that others can enjoy them.
The first thing people ask when you're making a Ren'Py game is "Pls add scene replayer", "pls add gallery", "pls add a way to name your save files" and people generally find it lazy when a dev leaves ren'py default UI there (which, to be fair, doesn't really matter as much since it works, and what counts the most is the game itself).
Still, a full fledged Ren'Py game will still require more UI (Screens), even just to have a characters screen.
Even maps are extra UI, a phone system is extra UI, an inventory, and so on.
Even a pure visual novel with no inventory, phone system, maps (or any clickables) will eventually have a gallery or scene replayer at the very least, unless the dev ignores any request.
Last but not least, they don't add any loading time. That's probably the thing that makes the least sense to me. This really sounds like you're throwing terms around x) which is fine, but no, that's not how it works.
-edit-
And in case you meant game size, that also wouldn't be true. Majority of a game's size is due to renders and audio.