He doesn't really have the 400m speed required to be an Olympic threat in 2021. His 2016 Bronze (also a PR for him) he got by picking people off who were actually running to win. Making that decision allowed him to place higher than he otherwise should have.
Carry that strategic intelligence into tactical Championships 1500m races, he could do some damage. He would also have some of the best raw speed among his 1500m peers, vs his current 800 peers.
He's also 25 and will be 26 in Tokyo. That's young, but unfortunately past 800m prime. That's a perfect age for an Olympic 1500m though.
Finally I think he has an easier road to the Olympics in the 1500m. Engels is the only lock. Centro is old. Gregorek is kind of meh. Thompson is kind of meh. Andrews is old. In the 800m he has to race everyone for 3rd, because Brazier and Hoppel will almost certainly go 1-2. If someone like Devin Dixon makes the final, it goes out QUICK and Murphy struggles when he opens with 50.X as seen in Doha.
I see him having more success and and easier path in the 1500.