I think the term "Legend" is thrown around way too often in MMA by fans. That devaluates that word. Machida is without doubt a great fighter and among the top, but he hasn't achieved anything that you really could label as "legendary". He is still somewhere in the middle of his career, so its still possible that he becomes legendary.
In my book, to be considered a "Legend" there are different factors. One are the achievements in the sport. I think becoming the champ in one division and only defending the title once is not nearly enough. Another factor is popularity, and by that I mean popularity beyond hardcore fans. You should be a name to casual fans and even somewhat known to people that are not fans of the sport. Ali for example, he is known to people through all generations, boxing fans and non-boxing fans alike. For MMA fighters that's a hard thing, because MMA is not yet known like other sports, in particular outside the US/Brazil and maybe Canada. When it comes to talks about MMA with non-fans there are only a few fighters that got recognition (usually without the name). "that Russian guy who never loses" (Emelianenko), "that tiny Brazilian who won against all the bigger guys" (Gracie) and recently sometimes "that Brazilian guy" (Silva). That's about it from non-fans.
We should keep in mind that we MMA enthusiats live in a bubble and people outside this bubble know next to nothing about MMA, but Legends are those also the average people talk about.