So they drive, or ship instead, and both being worse? or are you asking people to sacrifice their quality of life (i.e., _not_ take a vacation)? Because you know that won't happen.
In any case, taking a holiday or going abroad doesn't mean taking a flight, or driving. And I really doubt that taking a boat trip produces more CO₂ per capita than taking a flight, given that many more people will typically board a ferry boat, than board an airliner going the same distance.
The purpose of travelling for holiday is not always sightseeing. Sometimes (for me, most of the time) it's about what's at the destination and what you do when you get there. For example, a sporting event, a particular sporting or leisure activity etc.
On the other hand, I've also travelled by bus (no good trains) to go to a conference in Plovdiv, Bulgaria and I'd have also travelled my usual way to conferences in Athens and Rhodes this year, that got cancelled because of Covid. Like I say above, you can do this kind of thing at a leisurly pace, or you can do it fast.
But I think the real problem is that travelling like that limits your range. I was offered an exchange position in China but travelling there by land would have taken me a week or so, so I declined.
But I can accept that my options are curtailed this way. Air travel has spoiled us in that we take it for granted that we can travel to the other end of the world in a few hours any time we like (if we can afford it), but this luxury has cost us greatly in terms of environmental damage. So, personally, I don't accept that going at a sporting event or leisure activity trumps the need to reduce emissions from air travel.
So, to rephrase my original comment, above, taking a holiday doesn't mean you have to go around the world for it. You can holiday closer to home. So I won't get to see the Fiji islands? I can live with that. We're not going to get out of the mess we've done and still enjoy our way of life, one way or another (the "other" being that eventually climate change will make our current way of life unfeasible anyway; for example, Fiji will sink under the sea).
When I was a kid we were too poor to afford vacations out of the country, we drove to the nearest seaside every 2 or 3 years, during summer, with no AC, and I still had loads of fun. You don't have to go to _exotic_destination_234_ to enjoy your vacations.
People won't be able to drive from Berlin to Barcelona for the weekend in 3 hours for $50 (round trip), nor book a cruise from London to NY in 6 hours for $400. It doesn't take much to imagine how the world would look like actually, since coronavirus made most touristic places either very inconvenient or impossible to visit most people stay local and discovered that their region is just as beautiful as others.
These "what will people do without all these cheap vacation destinations" comment reek of entitlement