It looks that even here we already using @ Tag instead of quote, it looks a more clean way to discuss if we want to discuss something but we also want to reply to other X person without having several quote bars.
Adding the tag add-on to my site made a massive difference to block quoting and quoting to alert someone.
I even had to tell people to stop doing it as there was more quote content than actual content on every page.
So having Tag next to Reply may help even more with that.
Actually, I just tried a few scenarios related to this and I don't think it will work in the majority of cases.
Rarely do you tag someone at the start of a post so you either click Tag and reposition the cursor in front of it and start typing, or, you start typing, scroll up, find the post/user, click Tag, scroll back down again, and resume typing.
It's going to be quicker just to type @... in most cases I suspect.