I've been looking at a few carts and most of the closest to being actual games are right on the Lua code limit (but they have plenty of graphical / map space.) It seems that the real major limitation is the code size and not the sprite limit.
The code size limit is causing hard to read code and ugly shortcuts. I absolutely understand the reason for the limit, but I think it should be raised considering the code isn't compiled or compressed and a lot of carts have vast sprite spaces but no more code space. In most applications the code text doesn't cause any appreciable size increase. It's always the graphics and sound assets.
That said, I think there might be some interesting ways to fit more code in the size of that space but at what point does it circumvent the spirit of the Pico-8? Cutting variable names and comments doesn't make a game easier to write or eliminate design fatigue. It just makes it more difficult for difficulty's sake.
I'm not sure if the text size limit has anything to do with the .png cart format though.