Hmmm… It does essentially guarantee the legendary rank next season if you play enough even with 50% win rate, but it seems more of a convenience than a goal to me, to be honest.
Hmm… If you just want that ‘Legend’, then 11 stars are like… getting that mega-sword after you’ve already beaten that final boss, with nothing left to defeat with it — or, at least, nothing you wouldn’t have done without it; a ‘win-more’ bonus, if you will.
If the question is, what’s easier or faster: trying to get those 11 stars or just hitting the Legendary rank with 10 stars normally next month… I’m not sure, but perhaps the latter is.
As far as I understand, ‘MMR’ isn’t something abstract that exists independently of your ‘performance’. Actually, if it had (gone down) without you playing, i.e. after long inactivity, it might be even easier for you to get back — otherwise it’s supposed to mean a losing streak, more or less.
I dunno how exactly that ‘MMR’ works… In case of Elo rating, for instance, it corresponds to the expected win rate of one player against another and could actually be a good indicator, but in this game, those are just some hidden numbers assigned by a dubious algorithm — I dunno how to interpret them: say, what’d Elo rating in a game of roulette look like? And Hearthstone? 
Hit it a few times (EU, Classic), most notably — when climbing from the very bottom after years of inactivity, when every rank was like D5… After this kind of win rate, I suppose, they had no other choice than to give it to me. 
In suqsequent seasons, sometimes I’d hit it, sometimes not. But, as far as I know, it’s supposed to be related to a high win rate.
Funny: in my case, then, it’d be something I haven’t really aimed for, but hit it nonetheless.
Another one for me on the subject, though: clearing all of last week’s mythic boss rushes with a reasonable investment of time and effort on a case of bad design. In the end, cleared 3 out of 6, the first two being even farmable, didn’t bother with the rest… Ah, well, no matter.