It'd be one thing if Trusts had to be leveled up from the beginning, but they don't. All the way up until you finish the quest Shadowbringers, Trusts are always ready for the dungeon you currently need to run, and as I recall you can go back and run dungeons you've already cleared with Trusts after the fact as well.
It's only after you've cleared the aforementioned quest that this feature is stripped from you, and now you have to engage with an entirely unnecessary grind to "earn" it back. And thanks to the way Trusts approach dungeons, this is no small grind: It takes about three runs of a dungeon to level them up once, and each run will take around 20-30 minutes depending on how well you play. And because there's six Trust NPCs, you have to do this grind a minimum of two times—three if you don't optimize it by running them in a specific role. This adds up to a total of 18-27 hours for two Jobs, or 27-40.5 hours for three.
That is a massive grind, and the reward is lackluster: You get a title, the option to run with Trusts in a little over half of the level 80 Shadowbringers dungeons, and the ability to toggle between their old appearances and their new ones. The Story variant of the Trust system is also re-enabled, which is to say they give you back the feature they stripped from you after Shadowbringers. And that's it. Originally, it didn't even give you that last one—the option to run Holminster Switch with the Exarch and Lyna (the only dungeon at launch to have non-standard Trusts) was straight up gone for no reason other than to force you to engage the grind instead.
The grind also used to be much worse. Trust runs used to give around half as much EXP as they do now, making it a solid 40-80 hour grind depending on player skill. And at the time, there was no benefit to doing so other than running Amaurot and eventually The Grand Cosmos with your leveled Trusts.
Finally, I doubt they're going to abandon the Trust system in the next expansion. If anything, I expect it will be expanded with additional characters, like Lyse or whatever new/semi-new character will be the sort-of face of the expansion. Maybe they'll all be like Trust NPC #7, and start at the level cap. Or maybe they'll start at 81 and you'll have to grind them up to the cap yourself. But what about the remaining six? Will they remain at 71 if you never engaged with the system? Will they be locked to 80, with "new" versions of some or all of them replacing them in 81+ dungeons? Will you have to grind them up to 81 from 80?