If your players are anyhting like mine, worry not: they'll think of something.
That's where you-as-DM can help out: if it looks like they're considering a base, let them find a big heavy item to put in said base that provides safe reliable long-range travel.
In the game I play in, we have a big full-length mirror in our base that acts as a scrying device, and when scrying someone or somewhere you can then step through the mirror and be there. If someone "holds the mirror open" by scrying on you or your location, you can go back through it to the base; thus it provides two-way transport. The mirror allows us to have our base be in the middle of nowhere, which it otherwise is; though we also have an airship which also provides us long-range (if sometimes riskier) travel ability.
In an old campaign of mine, they found a big heavy tapestry of a pastoral country scene. If you stood in front of it and looked at it for a moment while thinking of a known location, the threads would re-weave themselves to show that location and you could then step through the tapestry and be there. That tapestry made their base (which by then they already had) much more viable as a long-term operation. The tapestry was only one-way, though; you either had to get back to base the hard way or send someone home to get the tapestry.
In both cases you had to have prior knowledge of where (or who) you were scrying; and both items only work to get to on-plane locations.
And-or, individual characters can always build their own strongholds, castles, temples, etc.
I've always had it that characters either have to train to level up or advance way slower (and advancement in my game is slow at the best of times!).
Yeah, 5e's cracks do start showing when one looks at how it deals with money, treasure, and magic. And the DM is kinda left hanging if-when she wants to fill in those cracks.
One option that your list doesn't hit, but that can provide a good and sometimes useful place for PCs to spend money (and time!), is researching and developing brand new spells, potions, and-or items. Spell research is obviously only something arcane casters will bother with, but researching and developing items is something any class can get involved with; and having your character's name forever associated with an item or spell is just plain fun.