This scenario is completely valid on your device and/or on a fleet of devices or systems, but not the market with a billion different users for market penetration. Plus, nothing is 100% secure, and so far they all have similar security levels.It's really not as clear-cut as that. Consider the following scenario:
I have a hardened OS and browser that is 100% secure. Adding anything at all to the mix will put my system at additional risk.
Of course, that is not a realistic scenario for something like an iPhone, which gets patched every few months (likely more so from iOS16+). But it's not that far off. You also have to consider the patching schedules of third party software. I've installed plenty of apps over the years that have not been updated in years. God only knows what risks they pose.
Firefox and chromium based browsers have a weekly update if not daily with serious exploits, apple currently do as you say uppdates every few quarters at most, that will hopefully be fixed in iOS 16 have . and then again if you use an app with sub 1% market penetration will likely be more secure than any software apple can create just from the nature of it not being worth their time. Find an exploit for a user base of 1 million or find an exploit for a billion users.