If Jay decided to leave at the end of May when he had his last show with the Tonight Show, waited several months to almost a year so his contract with NBC would expire and move over to Fox with his entire staff to do another late night show that would competing with Conan, Dave, and Night Line, my guess would be Leno's show would be drawing in the ratings. Let's face it, no matter if Jay had stale jokes, he knew he could bring in the numbers, and I'm betting odds if he went over to Fox after his contract with NBC ended, he would do the exact same thing, leaving the NBC execs to speculate if it was really necessary to let Jay go while making Conan the Tonight Show host.
According to Jay after a few days ago while explaing this entire thing to his audience on his prime time show, he was informed Conan was getting offers from other networks in 2004 to go do a late night show for them. The only way NBC could keep Conan from leaving was to offer him the Tonight Show within a time frame, which ended up being 5 years down the road. So NBC told Jay to let the show go to Conan even though Jay's ratings were #1. If that was the case, Leno should have just said to himself "to heck with these idiots who are forcing me out. Come the end of my contract, I'm going over to another network to develop my late night show".
But the only reason it seems why Jay said he would go back to host the Tonight Show is he has close to 200 people on his staff that rely on him for employment. If he had decided to leave along with Conan, forcing NBC to find a new host for the Tonight Show, I think everyone would have said Jay is a good man for leaving that crazy place. But even though Jay would be fine money wise, his staff would be unemployed in a state that has a higher unemployment rate than the majority of the enitire country, and I don't think that is something Jay wanted to live with. But he could be using that excuse to say it was an acceptable move to go back as host of the Tonight Show even though in reality he just really wanted his old show back regardless.
Personally I like Jay. However, he really should have left at the end of his era of being host of the Tonight Show to go to another network to do his own late night show. It would have worked out for him because everyone who missed Jay in the 11:35 time slot would enjoy seeing him again. Not to many people watched his show on prime time because either 1.) people forgot about it 2.) people had other shows to watch 3.) both 1 and 2.
With him going back to host the Tonight Show after he retired isn't good for his image. If ratings with him returning as host of the Tonight Show tank month, after month, after month and continue to do so, NBC is going to be stuck with their boneheaded decision and will suffer the consequences. They're not going to get rid of Jay but will have to suffer through the horrible ratings they forced onto themselves.
Of course that's my opinion if that scenario takes place.