> Now we live in an age of mass transportation where people routinely leave their towns to venture elsewhere

Future generations may grow up in a world of much more limited travel, existing in small areas of '15-minute cities' without cars, minimal access to air travel, living in tiny apartments, and most likely spending most of their time in some sort of metaverse, with very limited food choices. We'll have stopped burning fossil fuels and given most of the planet back to nature, and can't allow the working-class masses the transport/freedoms to access/enjoy/ruin it again.

If that's what the world comes to, it'll be a lot closer to life on a generation ship than the world we're used to today.

That world you describe is a dystopia and it would probably be better for humanity to wink-out at that point. Then again, maybe by then we'll have bio-engineered ourselves to have a native port into VR working directly with our visual cortex. Then again, by the time we get to the point we need to be serious about getting out of the solar system (100 million years) we arguably wouldn't even be the same species as today - especially as we'll have been bioengineering ourselves for millions of years by that point.

Not that any of this solves the problem of humanity having nowhere to go. The trip is ultimately futile since the stars you're going to are also dying.

> Yet they yearned for adventure

Some did. Most stayed in their little town for their entire lives.

> How is that going to work for your population confined to a ship the size of a small town?

The same way it worked for small towns before mass transportation was available.