CTRL+F hijacking is necessary in some cases when apps are not displaying the full text that the user would expect to search. E.g. when there's a 10K-line code file and the UI is not loading the whole thing into DOM, but the user would expect a "find" to search that whole code file.

They can have a search button for that, not hijacking default browser functions. Often I want both kinds of search.

Browsers can deal with very long documents. Ctrl+F works like a breeze on HTML that's 100K lines long.

Browsers only struggle to run heavy JS frameworks that wrap every line in a dozens of spans with dozens of handlers and mutate it all on every line scrolled.