I don't want to sound like I'm bashing Aldo. But lets look at his opponets
Mike Brown? yea where is he?
Faber? Highly overrated for beating up on WEC fighters for years. Many of his defenses were against guys who didn't make it or didn't belong in the UFC. Good win still, but Faber dropped to 135 and may not even be top 3 in that thin division. Faber never evolved.
Manny? How has he looked recently? Like a legit guy who fought for a title?
Hominick was a good win, but again where is he now? Why did he just fall off?
Mendes was a good win. He is still a top FW and has some of the best wrestling in the division.
Florian, again good win. But at 35 and rail thin Kenny wasn't the same Kenny as the lighweight version a few years earlier. He retired after that fight.
The division is corky. You got ok 155ers dropping (besides Edgar). Guys like Faber who really belonged at 135. Or guys like Mike Brown and Manny who really were never elite fighters with elite skills. The division is just becoming established a bit with an elite guy like Frankie dropping.