GP2 teams are insanely small compared to F1 teams, the only junior series team that could try to join F1 if they joined all their different series teams together might be DAMS. Other than them I dont see anyone with the manpower or money to make the jump sadly
Having some kind of two tier F1 would at least prevent a full on F1 team from having to finish last all the time. I mean, how long will Force India / Sauber and their sponsors be happy to stay in the sport once they are known as the 'back of the grid' and always coming in last. If they had 10 other Tier 2 cars behind them, the perception would be much better from Joe Publics point of view.
It's not just size, GP2 teams lack development and building facilities because they run spec cars. They could of course commission someone like Dallara to build an F1 car for them, but that's not going to be cheap either and sooner or later they'd want to have their own car development, so they'd acquire a factory and hire lots of staff - only to then get demoted back to GP2 the following year? I can't see how that would work in the long term. F1 is just too complex and expensive for lower tier garagist teams nowadays.
This is actually a decent idea, and would be a step up from GP2 for junior drivers.
There is a huge gulf between F1 teams and GP2 teams, we need a championship between GP2 and F1 that isn't a spec series so teams can learn to build a car, maybe we could call it 'Formula 2' ...