The thing is, the discussion here has been totally taken out of proportion.
Red Bull is successful at the moment, just like Benetton was back in the day. There is no denying that. Actually, the parallels between the two are frightening: pretty decent young German driver, designer acclaimed by many as the best, a good technical team having found a "groove" in the current regulations... Remind you of anything?
Now Hamilton's comments are not meant to pretend that Red Bull is not a "real" F1 team or that they are not as good / will not be as good as McLaren and Ferrari. He's just pointing out that one day, just like Benetton did, Red Bull will end up selling the team and will retire from F1.
There will be one day when F1 just does not make sense anymore for Red Bull relative to their core business: energy drinks. If Red Bull, as a company, sees a better opportunity to publicize their brand, they will take it. And leave the current F1 team in the hands of a new owner.
For McLaren, Ferrari and Williams however, Formula One IS their core business. If they left F1, they would simply cease to exist. And Ferrari has built its brand on F1, and has competed in every single World Championship since its creation. So those teams are not going anywhere.
That does not mean they are better than Red Bull Racing. But it does mean their presence in F1 is more stable in the long term. And that is the point Hamilton is making. He has grown up with the McLaren legend, and is now a part of it. His whole life, he has been a McLaren fan. No one can say that about RBR because they did not exist 10 years ago, and the team will probably have changed owner and name 10 years from now. That is just what happens in F1, owners and sponsors come and go, and generally the teams stay.
As for Mercedes, to me they still have to prove they are there to stay, unlike Renault who come and go regularly in F1.