I think the larger point is that builds would still work, albeit less good, with previous seasons, because they augmented the gear and you got the underlying gear transfered to the season. If there were “chaos gems”, or whatever that beefed up a unique, then at least you had that unique.
Similar, my Hydra Sorc build, can’t do the pit levels anymore he was in, but at least can still farm helltides and infernal in T4, so I can keep the char or transfer its build to another sorceror in Eternal, and things are still playable.
That just won’t work if you build up a Chaos Armour setup with 2 or more pieces, that build will be useless and all you can hope to gain from this season is mythics.