I still think Nader would gravitate towards the Greens or another left wing party later on. He'd be the bottom half of a presidential ticket who went down to a landslide defeat against a man who would wind up disgraced only a few years later. Even if he was elected to office, I think he'd be to damaged by his defeat to be a viable candidate for the Democrats in 1976. And if he refuses to support the party's later nominees, he'd not keep the support of the Democratic party, and probably not remain in office for long.
I do think this would make him a stronger third party candidate when and if he decided to run later on. Maybe he could get the Greens to five percent and get the matching funds they so desperately sought, leading them to become a stronger party. Likely though, this would have the effect of only increasing the suspicion of Democrats that Nader cost them an election, so we'd see any Nader run lead to the same situation as in our history, where it was good for the Greens in the short term but bad for them in the long term.