No, WE doesn't need an entire army. They should have just been a detachment that happens to allow WE specific units. Same goes with DG and TS.
Custodes? Never should have been an army. Space marines should have been close to the level of "eliteness"(regarding stats and point cost) of custodes, with at MOST custodes only being available as a random imperial ally unit to toss into an army for some narrative flavor. Imperial and Chaos Knights? Same thing, use them as an allied attachment to an army, not being their own skew mess of an army(that shouldn't even have separate codices, since combined they'd still be well under the total datasheet count most of those books are at). And again, I'd extend the same thing to the special snowflake loyalist chapters. Deathwatch, blood angels, space wolves, black templar, dark angels, did not need their own codices especially not in addition to getting access to the basic space marine detachments this edition.
Yet the loyalist snowflake chapters get to take whatever they want from the vanilla loyalist book. Honestly if anything, having to make every single thing it's own army fucks up balance more than anything due to making it next to impossible to ever do any reasonable amount of testing purely because of the number of iterations required.