Agree with the "the invincible few suddenly looking quite vulnerable"
Coming into 2013, Anderson Silva, GSP and Jon Jones looks to be unstoppable in their respective division. People are calling for superfight as this could be the only way they will ever get challenged. Remember how Dana "promise" that Anderson would face either GSP or Jon Jones in the beginning of 2013?
Look at how things has turn out by the end of 2013.
Jon Jones- The man who ran through former champions such as "Rampage", Evans, Machida, Shogun etc like it was nothing was looked human for the first time in his career by...Alexander Gustafsson! I thought Jones would dominate him via wrestling but it turn out that Gustafsson damage Jon Jones more than any of Jone's past opponent combined. In eyes of a some, Gustafsson even arguably beat Jon Jones.
GSP- At one point of time, many said he cleared out the Welterweight division. He pretty much dominate all his previous opponent and make it look easy (or boring). Then Hendrick comes along and I thought he will beat Hendrick fairly easily as well since I initially considered Hendrick to be a better version of Koscheck. How wrong was I. He took GSP to the limit and lost in a very controversial decision. GSP then announce his "retirement" for the time being.
Anderson Silva- P4P long time Kingpin. Like GSP, also cleared out his division time after time. Just when he thought his only challenge might be in LHW division or GSP, Wiedman comes in and snapped his 16 fight win streak (something that happens due to Silva own antics.) In a rematch, suffer a gruesome leg injury that might very see the GOAT gone for good.