Research wrote:
I'm procrastinating on my work so looked up what happened to every 1500 winner since 1980 in their subsequent games. Tl;dr Centro was the worst of former champs who made the field, but only about 50% even qualified in the first place.
2016: Centrowitz - placed 14th in 2020 semis, did not make final
2012: Makhloufi - won 2016 silver
2008: Kiprop - placed 12th in 2012 final
2004: El Guerrouj - retired, did not contest 2008 Olympics
2000: Ngeny - did not contest 2004 Olympics
1996: Morceli - did not contest 2000 Olympics (though did compete in 5,000)
1992: Cacho - won 1996 silver
1988: Rono - did not contest 1992 Olympics
1984: Coe - did not contest 1988 Olympics
1980: Coe - won 1984 gold
Also notable is that every single one of those men who actually made it to the Olympics and defended their titles was younger than Centrowitz is now (31 years old, turning 32 in October). In fact, I could only find ONE instance of a defending 1500 meter champion performing better at the age of 31 or older. That was Kip Keino in 1972, when he followed up his '68 gold with a silver at the age of 32.
We can argue whether Centro underperformed based on his current fitness, but even in that regard, I'm not sure he did. Just based on his 3:33 1500 and 3:49 mile, nothing screams "top 8 runner who blew it by missing the finals". Maybe he mistimed his peak and travel/adjusting to Tokyo time zone, and will run a lot faster later in the season - who knows. But overall, it looks like his level of fitness matched up decently well with his semifinal race. Experience counts for something, but if you are a 3:33 runner going up against 3:27-3:30 guys, there is only so much you can do.