As was already mentioned, depending on model, GK RB heads can pretty much be found anywhere on the used market for between $300-400. There are just that many of them still up and running in the wild, and at that price IMO they're a steal - and why I currently have 3 of them in my possession.
I've personally never had an issue with any of mine, but if I did, there seems like there should be enough donor amps, or just outright replacement amps out there in existence that I really don't ever see myself being unable to get my fix with one.
Hell, while 800RBs - particularly those in good cosmetic condition, have gone up in price recently, that is an amp from the 80s! It's been over 35 years since it was released, and while I admit that new replacement parts for it are definitely more difficult to source, there are still a ton of the amps themselves in existence.
I guess that's a long winded way of saying that as far as GK being the consensus, they're definitely deserving, but I think a lot of that just comes down to their sheer number. BUT there wouldn't be so many in rotation if they died every 5 years!