Yes I've definitely noticed that crash in multiplayer population. I suspect a lot of it is from people not being viable in multiplayer. The people who brought their legacy top tier mythics into new battles and got smeared by new legendary, not to mention new mythic, mechs who didn't want to open up their wallets to continue to be able to compete surely quit. Those that remain, well they face the new nightmare of the mythic with fully upgraded new death punches:
I fought this thing the first time I saw it, and when it destroyed my 300+ energy pool in a single round and does more damage than my physical weapons while doing so, well you better believe I quit on the spot when I see that now. So now your remaining population is further decimated by players who don't want to play. I imagine someone who paid enough to get full new mythical had a good laugh at first seeing everyone instantly quit when matched against them. But that laugh has turned into bitterness as they release no one wants to play matches against them.
Heck even my Legendary Mech generally seems to be leaps above most of the players I fight, and I've been max stars rank 1 for many many months by now.
These are the kinds of problems that won't fix themselves.
This league solution will at least fix the problem facing the remaining playerbase. And hell if you really wanted a larger player base, what ever happened to getting Super Mechs on steam? That's a huge playerbase booster, though it's only a temporary population injection if your game isn't good enough to retain your Steam players. But that's a story for another time.