Exactly, Stun is just more than “lol you can’t move”.
It presents interesting situations, almost every match against a reaper is a game of chicken on when to use the stun or when they will use Death Blossom. If I get pressured or an opportunity to get a kill by using the stun presents itself, should you use it?, even if you know Reaper has Death Blossoms?
Also Reaper has to be aware of where Brig is and if she has used the stun.
If they remove that interaction, Reaper can just go “lol ult” without a worry, well that sucks.
And that is just ONE interaction that is going to disappear (In the hypothetical case that ults can be interrupted in OW2).
If Brig is just a another Moira-esque support, it will be devastating to be honest.