I think this is a smart play. Perhaps Usyk actually wasn't giving his all against Verhoeven and now he's created demand for Usyk/Rico 2 and can divert attention from Kabayel
This kind of thing can happen but it is very unlikely here. I think it would usually take the form of carrying a non-threatening opponent. Being behind after 9 rounds against a guy you have only hurt in one previous round and who hasn't slowed down yet is very risky.
No he simply became an old man in the ring at his advanced athletic age of 39 years. It happens to every athlete with many boxers finding the same old age in the ring when in late twenties/early thirties.
If there is a rematch Usyk should win with relative ease. He'd be older but: 1. Rico isn't young himself and just got TKO'd, so he added more wear and his confidence should be lower now his 11 year win streak has been ended 2. Usyk has experience of Rico's style now, and 11 rounds with him to study 3. Usyk has the knowledge that even on his worst night, he could still drop and stop Rico, and Rico knows this too 4. Usyk would take Rico more seriously in a rematch with a point to prove, coming in better condition physically and mentally 5. Usyk has had three pro boxing rematches and won all of them, with fewer problems in most cases