To a user on the outside I can see how it seemed the site was perfectly fine as it was, especially If you didn't use the private message system as your primary source of RP's, however PM's are most certainly the majority of the traffic here on BMR. It was invisible, but the archaic system of the old format was quite simply not built for it. Imagine in your mind a large exploration vessel. Picture it however you desire; an old seafaring ship braving the vast unknown of an ancient fantasy age, or perhaps a fantastical construct built for the terrifying expanse of space, navigating the outer edges of the universe. Now picture that same ship as wonderous as it is on the outside, being held together on the inside by tape and staples. Sure It's still doing it's job and on the outside or even on deck everything seems fine, but underneathe it's coming apart seam by seam and leaking water or space or lava or what ever you had going on there in that mind palace of yours.
Stepping away from my imaginary metaphor fiesta, it'll echo what Sigr was saying. The upgrade was needed. A typical user may have been none the wiser but in the backend, it was very noticible by staff. There were countless glitches and it was getting worse. There was a ton of bloat because capacity was at it's limit, being supported by tacked on alterations. There's more to it than that, but that's the gist of it, it was needed, and there's now a lot more quality of life changes for both users and staff a like. Yes it's different, and even off putting for some. Admittedly it's taken time for staff to adapt as well, that's including my self, but it is what it is.