The "who cares if it doesn't scale" attitude sucks if your users suddenly pass the threshold for whatever reason.
There are also photo apps where you can scroll through decades of photos.
Also, text editors should be able to handle large files, and filesystem widgets should handle large directories.
When you pick a UI framework you know how your app looks and what 'complexity' means for your UI. And you make an informed choice of which framework you choose based on that.
Or one would hope you do. As that's part of the job of being a software engineer.
If you give that decision to your clanker and it chooses egui a for spreadsheet app: that's still on you. At least for now.
But it's not on egui, either way.
Experience shows that choices made in the beginning of a project are very often reconsidered later when requirements change. In those cases it is nice if you choose for the worst possible outcome, so in this case you choose for a high performance GUI framework which can scale.
Obviously, today you can just write "please rewrite this badly scaling app with that scaling GUI framework" but I think that's outside the scope of the discussion.
The fact that you can just prompt a UI kit swap should be at the core of the discussion then; wouldn't you concur?