Of course there are unreleased 0-days, but you can't do anything about it. Most of them are even kept secret by companies that sell spy software. However, public 0-days are way more dangerous because they are being exploited actively.
Using a different browser until a particular issue is fixed when you are e.g. a journalist still helps with getting hacked.
The difference is: Microsoft never forced, they just nudged users very aggressively. They got into trouble for that multiple times and needed to adjust their practices (but keep trying). For some reason, nothing ever happened to Apple (yet). In my opinion, tech media is way too lenient on this as well.
This is not just bad because of privacy, Safari has been slowing down progress on many web apis for years. Other browser implementations would also probably be faster and/or drain less battery. I could probably come up with even more reasons.
Actually it does, because you have options if a 0-day surfaces. Your logic only works if there happen to be multiple 0-days released at the same time on all major browsers which affect all recent versions for each browser (because on iOS, you can't even downgrade to a previous version that could be unaffected). That will probably never happen.
Ah, now I got what you meant. I was just suggesting switching temporarily while the published 0-day would be public and unpatched, because this is the time in which the issue would be exploited the most.