Will old legendary raids ever return?

Eventually, I think they will. Maybe during events, or something. Also I'm guessing that when they make a return, their shiny forms will also be released as a way to get people who already have a bunch of them to want to raid them once again.

Niantic brought back Articuno, Moltres, Zapdos, and Lugia for two extra weeks once the week for Zapdos raids was over during the summer. It's definitely possible the other past legendaries can make a return, but maybe for a shorter period of time rather than a month.

Yes, I am fairly certain they will return at some point.

I don't know how many legendaries there are in the original games, but at the moment we're getting a new one every month, so surely Niantic will run out of legendaries at some point?
The option is then to either stop Tier 5 raids or bring back old ones.

But even before then we will probably get to see the old ones make an appearance, but perhaps for a short time only. Perhaps get the three birds during a couple of weeks or so.

Anyway, I think we'll see them all again at some point.

It would be a big mistake on Niantic's part not to bring them back. Why would anyone new want to play a game with the motto of "gotta catch em all" if they could never catch em all?