If oneplus worked with Google to put vanilla android | a fully supported bug free version | onto the OP3 I'd consider it, but the form factor would still likely be a deal breaker.
I've run CM for a few years on it actually, decided to try Paranoid Android again recently when it looked like they started back up (I missed HOLO) but that now seems abandoned again and I really missed some features that turned out to be from CM, so I'm probably going back soon.
It really seems like inexpensive high-end phones should sell like hotcakes, but in practice, it doesn't happen. It's arguably very possible that a $650 phone will sell better than a $450 phone even with the same components, because it'll get perceived as "high-end" in a way the inexpensive one wouldn't.
Heck, look at the OnePlus 3 right now, it has more battery than a Pixel, an 820 instead of an 821, but also 2 GB more RAM than the Pixel. Pretty close specs overall, but one device costs $250 USD less. Unless the only thing you care about is camera quality, you really have to consider if that Pixel will be $250 more worth of phone than the OP3. Is that 820->821 upgrade going to make a difference in your daily usage?
I think the answer is probably not, and I think that holds true for other flagship devices. Never will I pay $650 when I can get a roughly comparable phone for $400 or less.