The traditional UA fakery (adding Mozilla to the start and then just tacking on browser engine names) was the result of outdated websites breaking browsers.
The problematic fakery here is that bots are pretending to be people by emulating browsers to prevent rate limits and other technical controls.
That second category has also been with us since the dawn of the internet, but it has always been something worth complaining about. No trustworthy tool or service will pretend to be a real browser, at least not by default.
If AI agents just identified themselves as such, we wouldn't need elaborate schemes to block them when they need to be blocked.