Quite a few people think that, but I'm not sure why, at least in the case of Spirit.

Indigo sold its remaining, and fairly substantial, holding in Spirit immediately before buying Frontier, which seems an odd thing to do if they were planning to merge the two.

Allegiant? I don't see it, but anything's possible. They're two quite different models, and according to a post on another website, based on DOT stats, Indigo's Frontier made more money ($103 million) in the first nine months of 2014 than Allegiant made in the whole year ($87 million).

Mostly, I think Bill Franke at Indigo wants to be boss and if Indigo can turn Frontier into a money printing machine, as seems to be happening, and if they wanted to merge with anyone, Sun Country Airlines might be more viable.

In any event, I doubt anything would happen until after Frontier has had a public offering, which Indigo isn't even hinting at yet.