I don't have estimates, but I think it's safe to estimate that there are millions of otherwise perfectly valid phones that either are in use, with serious vulnerabilities (in particular, bluetooth ones) and are not going to be patched, or have been thrown away because of that.
Most of the vulnerabilities require you to be loading sketchy apps, many of these are checked on Play Store by scanners, so unless you get very unlucky or sideload sketchy apps, probably not a concern. Web view and most other network exposed components are updated from play store, since stagefright they sandboxed all codecs, etc.
It's really not that bad for a typical user to just run an unpatched phone, certainly better than a Windows machine with local admin where downloading and running one executable is all it takes. At least on an unpatched Android device they get some level of sandboxing.
There have been large set of attacks against modems and media decoders that do not require any kind of app installation. iMessage is a constant source of iOS CVEs. Media codecs are a constant source of headaches in the Android world as well and they require a driver update to fix (since most of them are HW decoders).
Pretty much all of the big security issues in the last year required no user interaction on Android or iOS to exploit.
In addition to the sibling post, Bluetooth is another attack vector.
A phone I had to abandon in the past was a Nexus 5X, which I believe has a set of very serious Bluetooth bugs. I don't remember the details, but they were very easily exploitable (as in: either doing nothing, or just keeping the BT menu in the foreground, which is a common operation).
Sure, if most drivers were open source like on desktop linux we could tweak them to work with newer and newer Android versions, but what happens when AOSP gets a computationally expensive update (i.e. embedded ML voice recognition or realistic TTS)? then you would have to create custom versions of AOSP that only have the stuff a particular phone model can handle and this would create and unbareable ecosystem of poorly maintained distributions
You disable those features on lower-spec phones or fall back to a low-resource alternative. If I'm on an older device that can only run eSpeak instead of a more realistic but expensive TTS engine? Fine, I'll run eSpeak! You don't have to bundle everything together into an image for each device. If you want to disable stuff, do it at runtime.
Or if you really need to remove stuff due to storage limitations, have a small manifest that can be used to build an image with unused things excluded. This doesn't have to be "oh we need to make completely custom versions of the OS for each device", this is basically "install these packages, don't install these ones". Modularity!