Magical Mystery Cure as the series finale would have been egregious. Although MMC had some elements of a series finale, it opened up new doors and left a lot of unanswered questions. If MMC had been the final episode, we would never have gotten to know if Rarity achieved success as a fashion designer, if Rainbow Dash finally became a member of the Wonderbolts, how Twilight ended up doing as a princess, etc. There were also many things to look forward to, such as the Equestria Games, or Discord using his chaotic powers for good. We wouldn't even get to see Rainbow Dash and Scootaloo's new relationship. MMC was also really rushed. With a series finale, execution matters a lot, and the last thing you want is for your series finale to feel rushed. I feel like if they drew the idea out through a two-parter or an hour long episode, and changed a few things, it could have worked a lot better. That's not the case though, and the MMC that exists would have been a rushed ending with no closure whatsoever if it had been the absolute finale. It's like you're a little kid listening to a story your grandpa is reading, and then your grandpa unexpectedly stops, gets up, and just leaves.