I am highly skeptical of claims toward the OSS router firmware scene being less useful than manufacturer provided ones. You're right that speed to support new features lags in OSS, but who cares? I buy the router based on the hardware compatibility list, not the other way around. Reliability and longevity is a lot more important to me than the new shiny. You're also right that today it may be difficult to meet all the requirements with open code, with AC support being a sore point. I'd use that as a reason to delay the purchase until i can though, not as an excuse to head any distance back toward less open development models.
I still have two Linksys WRT54GL units left in operation. Long after Cisco/Linksys stopped worrying about that hardware, I was happily served by the software communities around DD-WRT and then Tomato. Manufacturers like Ubiquiti are useful to me to the extent they embrace that philosophy. In the last year Linksys seems to be moving back in the right direction again. We'll see how that plays out.
I'm also skeptical that having two points of failure in a network can ever be more reliable than one, which complicates your flexibility argument. Whenever I decouple routing and wireless onto separate boxes, problem resolution is harder compared to having a single unit to swap out. One of the reasons I ended up with so many cheap WRT54GL units is that I could easily have a spare with a duplicated configuration for every install. At any scent of trouble, I just replaced the whole unit.