Yes the roster needs a major overhaul, and Poch said this two years ago.
I think a good portion of the players ran out of gas, more than quit on Pochettino. The summer before, during the 2018 World Cup, several of our players went deep into the knockout rounds with their countries (Kane, Eriksen, Lloris, etc...). Then you go right into the start of the 2018-19 campaign and go on an extended FA Cup run, the EPL matches being re-scheduled for our CL run started stacking up, and you could tell that both clubs in the Champions League Final were running on empty. Liverpool didn't look great either. Spurs had zero depth too, after going nearly 18 months from signing anyone in a transfer window. Lucas was the last signing in January of 2018. Kane was hurt, Son was hurt at times and suspended at one point, Dele got hurt, others were in and out of the lineup, and the squad was just fatigued. Poch couldn't rotate the lineup due to the lack of depth.
We had several guys that just came into the 2019-2020 campaign with a lot of minutes on their legs. It wasn't but a handful of weeks after that CL Final that the international friendlies began, and then the EPL season. The way Poch runs his training sessions, I'm sure those guys were done. They looked it during the first dozen games. By that time, Levy had made up his mind on Mourinho.
I think you're right. Add Aurier into that mix too. Look at how Mourinho handled him when he left the stadium; zero accountability. Contrast that with what Arteta did with Auba last week. I'm sure that didn't go unnoticed with Dele. Then Aurier moves right back into the lineup.
As for replacements, Rogers would be good and so would Nagelsmann. As long as it's not boring, negative futbol I don't really care at this point.