We hear you. Given what happened last time and everything.
But this time is different. Leno can't take his job back from Fallon like he did from Conan O'Brien — we should probably add a "probably" here, since he is still Jay Leno — because his circumstances have changed.
In 2009, when O'Brien replaced Leno, NBC was eager to keep both men on the payroll. O'Brien was contractually signed to "The Tonight Show," and NBC kept Leno from running off to another network by signing him up to host his own 10 p.m. show. The arrangement was the result of a five-years-in-the-making transition plan for "Tonight" that, well, didn't work out as planned.
This time around, Leno has no contract after fall of 2014. That means he will only remain in NBC's employ for a few months after Fallon takes over in February.
So the only scenario in which he could reclaim "Tonight" would be one in which "Tonight" ratings collapse with Fallon, and Leno again volunteers — as he did in O'Brien's case — that he'd be willing to take "Tonight" back.
That would mean NBC pulling the plug on Fallon about as fast as it did on O'Brien (pictured above with Leno in 2003). It would also require it to start from scratch on a new plan to draw younger viewers to "Tonight." A big part of Fallon's appeal for NBC is the hope that he can keep ABC's Jimmy Kimmel from getting a lock on young audiences.
It would probably be easiest for everyone if Fallon can score well, early.