That's a bit too coarsely grained IMO. There are different ways to resolve the question of security vs usability, varying by threat model and user confidence.
Graphene's approach is not inherently untenable: using the play store can be done via a separated profile - which is probably good opsec anyhow.
An alternative approach (which I believe the Graphene team are aware of, but don't necessary encourage) is to use F-Droid or the Aurora store. I believe there are some important shortcomings with this approach though.
In regards to 3rd party stores, I wouldn't use this approach for any kind of corporate or professional application, but if you're an end user there might be a case that this is preferable (suppose privacy is a much greater concern than security). I think that would be case by case though.
In the case of F-Droid, the apps need to be built using F-droid's build system, and is signed using F-Droids own keys[1], rather than the devs signing themselves. Not awful, but definitely not good.
So, regarding the 'too hard, gave up' problem you mentioned, I think you're right that its an important consideration, but disagree that Graphene's approach is "strictly" worse third party support.
[1] https://wonderfall.dev/fdroid-issues/