Hold on, metro/subway capacity isn't the issue being addressed by the parent comment.
You raised the problem of metro stations being further than a walkable distance on either end of the commute.
If a subway or train service is at standing room only capacity right now even before you help new people to get to it then it's kind of a moot point anyway; you need more capacity before you can add more people.
The economics of train routes are dictated by how many paying customers you can move per hour. If everyone carried a portable bike, you would increase the potential user base of the routes, but decrease the customers/hour. (When trains are near full, and to be economically viable, they need to be, loading/unloading becomes a major bottleneck and everyone carting around a portable bike would really slow that down.) Even if you build new routes for those people, the amortized cost per user you can fit in the train is going to go way up.
Who said anything about everyone using a foldable bike (all bikes are portable)?
Your first argument was that people couldn’t use public transport because of the last mile issue. That only affects a subset of passengers and is answered by folding bikes.
Your second argument is that it would impossible to operate a subway if passengers were allowed to bring a second cabin sized carry on bag on (that’s roughly how big a folded Brompton is).
Well there’s already plenty of examples where operators are already doing this successfully.
The London Underground for example operates just fine allowing folding bikes to be taken on at all times of the day.
You raised the problem of metro stations being further than a walkable distance on either end of the commute.
If a subway or train service is at standing room only capacity right now even before you help new people to get to it then it's kind of a moot point anyway; you need more capacity before you can add more people.