Regarding TATL travel, there are a few more aspects than what has already been discussed.
1) There is some not-insignificant amount of "pent-up" VFR-purpose related travel that is posed to unlock as soon as its feasible in enter respective countries. I personally know many people that will get on a plane to the UK and Germany as soon as its feasible without onerous quarantines or testing regimes. Once the countries can figure out how to make it work, there are people that have not seen parents/siblings/children/grandchildren in almost 2 years now.
VFRs, at least those with a non-U.S. passport have been allowed to travel and have been traveling. Like my wife and everyone else I know with an European passport who went to Europe last Summer. The UK was also allowing US passports in, last Summer for a period of time. A friend of mine with an Austrian passport just went to Austria on vacation last month. I'm not really sure how much pent-up demand there is but for me personally visiting family in Europe this Summer is out of the question. And I always knew all along that I was allowed in. I just don't want to deal with all the Covid restriction hassle. I can do that in the Fall or at Xmas when we have more clear traveling rules and hopefully less restrictions.
The biggest problem right now is the complete lack of information regarding the rules and when they will change. I agree that most Americans have already made their Summer vacation plans by now. I'm sure there are a small percentage that is waiting and hoping that the travel restrictions in Europe are lifted. One of the biggest ones being the testing requirement that is still in place where most countries charge upwards of 100 Euros for a PCR test. That on top of an expensive last minute fare, on top of still potential Covid restrictions at the destination, on top of kids not yet fully vaccinated, will keep a lot of people from flying TATL this Summer.
Typically European summer vacations end much later (mid September), than U.S. school vacations so there might be some hope for Europe originating leisure traffic. But I'd say the ship has sailed for US originating TATL travel.