For most 20-30yo, the 9-5 lifestyle is absolutely expected to be there for the next 30 years. Vacations are often handled by traveling, visiting extended family, etc. There normally are many options available. And by the time retirement comes, children will have moved out, or perhaps the parents will have sold the flat, or left it to the children, and moved out elsewhere. I don't think there is something as a "lifelong estate purchase" these days in the cities. Not when you're in your thirties.

> The elephant in the room: if you can't tolerate your partner in the same flat/house for longer periods of time then IMO you shouldn't marry them so... ¯\_(ツ)_/¯

Easy to say :). But some people only discover that quite late - once some unpredictable circumstances (like job loss, or a global pandemic) force them to actually stay in the same flat for long periods of time. Most working-age adults don't experience that ordinarily (and I think every couple expects they'll work such issues out by the time they get to retire).

> But what you describe just seems... more like a good roommates' arrangement / contract, and not an actual relationship?

Relationships are complex, and every one is different. They get even more complex if extended family gets involved, or when children appear. And sudden events like a global pandemic have a way of revealing previously unseen cracks in the foundations. If people were aware of all this up front, humanity would have died out centuries ago :).