Watch FOTC, and Thrilla side by side, then come back and tell me with a straight face Ali didn't decline at all. Frazier as well.
Yeah, 'that' low. Just to compare, I have it's counterpart, The Thrilla in Manila in the top twenty, maybe top fifteen on a particularly good day. I have Gavilan vs Turner just behind it at sixty, and Zaragoza vs McCullough just above them. The criteria I used for my top one-hundred fights was essentially that the skill level and greatness of the combatants were irrelevant. What I was looking for was just how enjoyable a brawl it was to watch, and I don't find heavyweights quite as enjoyable as I do every other weight. So the seventh most enjoyable fight there (IMO) was never going to rank highly on my list. But please, don't let me give off the impression I don't like the fight. I still have it quite high at heavyweight (can absolutely see it higher, too), and even at fifty-nine, that's still makes it a great, great fight. I love Frazier, and this being his best performance means it's a fight I often revisit. I just don't rate the action in it as highly as others do, as I do others, but even still, I have it top sixty. That means I rate it very highly as a fight.
I'm still compiling my own personal 100 and am still only on 90. The only criteria is how enjoyable I found a fight, so it's somewhat idiosyncratic as I might not have been in the right frame of mind for it when I watched it. Having said that, anything in my top 25 or so I've seen multiple times anyway so they are definitely my go to fights when I just want to watch a fight I know I love.
The thrilla, whilst a classic is down below a good few heavyweight battles for me. Historically important but both men well past best.