But they can only buy them if the carriers are offering them
True. But it will be a buyers market. Cash and credit will be king. Sellers can chose to meet the market, or can decide not to sell.
And any airlines that have received generous government support will likely be expected to "do the right thing for the country" and encourage tourism.
That's a pretty brave prediction, Slats11!
Even in 2021, some people will be travelling. Even if holidays are not approved, some people will seek approval as family business (helping elderly relatives arrange affairs) or essential business travel or whatever.