This is interesting, but also a mixture of truth and myth. In 1970, 3.9 players per team played 140 or more games, so there were not typically 7-8 position players playing 140+ games. There was always a lot of platooning and shuttling and injuries even when some of us old guys were kids.
In 2018, it was down to 3.5 players per team, so not really a dramatic difference over the decades.
There was a big change starting in 2019 when it suddenly dropped to 3.0. Last year it was 2.8. In between, there was the covid years. This is a significant change. but it's still just a difference of one player (four players per team versus three players).
It's also a new change, so I don't really know if it's a good idea yet. Is it leading to more productivity? Fewer injuries? or is it just a way to save money since part-time players make less money than full-time players? Is it easier to stay under team budget if you have more part-time players?
I do agree that the roters should be expanded.