…in multiplayer content.
There will very likely be a final “catch up” event a few weeks before the next expansion where you can easily gear up your 70s to a higher level quite easily.
It won’t be heroic level, but it’ll the best gear flowing for a single player. It will also likely be a good time to finish leveling some toons to 70.
And, yes, to your point about “why bother”, that’s very apparent this expansion contrasted to earlier expansions.
Combining the “borrowed power” aspects of the earlier expansion, plus the story elements, there was actually motivation to level and gear alts. For example, while in Shadowlands you could certainly run a single toon through all 4 covenants, there were certainly incentives to run 4 toons through them.
But in DF, we have no borrowed power, we have just the single story. As far as storytelling and “content”, there’s pretty much nothing different that you would see leveling and gearing a new toon.
Obviously, if you just like gearing toons, and running the content (M+, Raids), then that’s its own reward. But if not, no real reason to do much more with more than one character.
To that point, since gearing alts using the catch up mechanics is so fast (what was Forbidden reach, what is now Time Rifts and Dreamsurge, and who know what it will be in 10.2), then there’s not much reason to gear them. You may as well park them until the expansion is almost over and wait for the final event.
But, if you like, it’s probably worth while to take some time and get them to 70, regardless of gear. Then at the end, run them all through the last event, gear them up, and have a fresh squad waiting for 11.0.