trust boosts?

[BRVV]

I think the reason is more SE knows people would be pissed if a new expansion came out and you literally couldn't use trust on new content until you run the previous expansion dungeon 3 times. Skipping the lvl makes it so it can stay in line with the actual lvling content.
Goes in line with the whole respect peoples time thing everyone always talks about.

Except they didn't get boosted for EW. So yep, you can't use trusts for EW dungeons unless you have at least one of them leveled (thanks to Graha and Estinien starting at 81). What you mention is also a non-issue since you can simply use duty support.