de0u For example, I think that the Twitter/X app is popular; there has been a fair amount of traffic here about the app not working, then working, then not working, without Play. Maybe it's working now? There is also a lot of traffic about bank apps.
High likelihood must result in proportional amount of relevant discussions.
The trick here is that "absense of play services/store" is universally unique for all users. You either have it installed or not.
If an app (assume certain version, one of latest, people eventually update apps) can't work without Play Services, it will fail for everyone (99% prob).
So "high probability" would be reflected in strong "hey guys, if you plan to use more than just few apps distributed via Google, GOS will be unusable for you without Services".
However, here on the forum, majority of app-failing threads are a side-effect/reflection of the type of community we have here around GOS.
All those permutations of various networking layouts (VPNs and their kinds - free proton, non-free proton, cheap VPN, Tor,...), profiles schemes, private spaces, upstream DNS adblockers results in a typical app-failing thread "- hey, why the app X doesn't work? - strange, it works for me in owner profile on Mullvad VPN".
It would be much more even here on the forum, if it would be indeed "high likelihood", like at least 50% of threads would be "- why app X doesn't work for me? - you must have Services installed/enabled".
Subjectively, it's no more than 15-20% like that.
In my particular case with my apps distributed via Google (few fins, gvmt, taxi etc), I'm lucky, it's less than 5%.
They may wine, but never fail.