I think this is probably a lot of the value of YAGNI.
The more crap you add the harder it is to fix bad architecture.
And the crap is often stuff that would be trivial to add if the bad architecture weren't there, so if you fix that you can add the feature when you need it in a week.
I think that's probably part of it; but on a really simple level with YAGNI you're not expending effort on something that isn't needed which reduces cost.
What I try to do is think about the classes of functionality that might be needed in the future. How could I build X feature in a years time?
The more crap you add the harder it is to fix bad architecture.
And the crap is often stuff that would be trivial to add if the bad architecture weren't there, so if you fix that you can add the feature when you need it in a week.