I'd be willing to put money on the general person never needing more than a small portion of their library but for the rare exception, at which point a free Scroble-ing service or YouTube suffice. If you reflect, are you really going back to the depths of your music library to find track 11 off that one less-popular (insert band name) album?
It's simply the philosophy that most of the stuff we keep with us on our TB hard drive laptops isn't actually stuff we need to keep with us.
Obviously Apple considers your philosophy to be an edge case to their intended business model and would rather not offer you moderately priced options to upgrade, which is certainly a shame.
Sure, but that's not the same every day. I don't want to shuffle my music collection around constantly; I'd rather just have it, so if on a trip I'm reminded of $bandname and want to listen to an album, I don't have to futz with downloading it on hotel wifi. To me the cognitive overhead of manually operating some kind of caching scheme, where I purge rarely used files to the archive and then have to page-fault them back in again, is not worth it.