and yet we've seen it. I would argue it is a combination of factors i.e. health, opponent strength, and loss of talent (both by health and losing better defenders in the off-season). I do think health is probably the biggest part of that but the other things can't be discounted.
To your East vs West argument:
This season, the Celts are giving up 111.9 points per game against Eastern Conference teams, as compared to 110.5 points per game against Western Conference teams.
However, the Celts have a 16-13 record against the East, compared to 7-12 against the West.
Last season, the Celtics gave up 106.7 points per game against the East, while giving up 108.3 points per game against the West. They had a 30-13 record against the East, compared to a 18-11 record against the West.
So I think there is some support there for the idea that the East has gotten tougher, and it's having an impact on the defensive rating. But the gap between how they're defending against Eastern teams compared to how they're defending against Western teams is about the same, it's just flipped.
Also, if the East becoming stronger were having such a big impact on defensive ratings, you would expect to see that across the board. On the contrary, 3 of the top 5 defenses in the league this season are in the East. Meanwhile, 6 of the bottom 10 defenses in the league are in the West.
Instead, what I notice when I look at these rankings is that there are two Eastern Conference teams that were top 5 in defense last season and that are now solidly below average: the Celtics and the Raptors.
Both teams have been hit hard by injuries and COVID this season, and both teams played for a long time in the bubble last season.
I suspect that's where the biggest piece of the explanation for the defensive drop-off can be found.