I just believe his game has been figured out. He's gotten older, the injuries are there and he hasn't fought in some time.
Fighting is a funny game. He could pop a cat on any given night.
With younger cats getting in, studying his tape and knowing his injury history Lesnar is a beatdown waiting to happen.
I definitely don't think he was "figured out", he was sick dude. Now diverticulitis is 5,6 years in the rearview.
You don't just "figure out" how to stop a 290 lb HW Div. 1 NCAA Champion from power doubling you onto your ass.
as for arguing if top 5 matters, how about this? He would be the #1 draw and likely make 1 million in base salary for every fight? And anyway, yea, being top 5 in your weight class is pretty impressive considering there are thousands of fighters in thousands of orgs across the world. Being behind only JDS, Cain, Werdum and maybe a couple of other dudes is impressive. Another draw to MMA is competition. Brock is a natural competitor and you don't really get true competition in the WWE
I'm not saying it is likely, but just know that Dana White and Brock have a great relationship, have been in talks and the UFC would KILL to get him back. So the possibility is real, even if 2-3 years ago I didn't believe it.