I haven’t seen any numbers to validate what is occurring. But transfer, don’t sit existed when my now 35yo daughter played college softball. On a year to year basis the number of kids desiring to play college ball isn’t changing. Transferring doesn’t shut players out. The portal is affecting roster stability. A less talented, marginal kid who gets discarded might decide he’s had enough rather than transfer.
My son played when players had to sit a year when transferring. He won a starting position mid season freshman year. He returned soph year to find a JuCo All American at his position. He wasn’t forewarned this was occurring before hitting campus in the fall. The coach told him chances are he will play somewhere. He did. He was an athletic, seven position prospect.
What did affect the numbers was when D1 rosters were limited to 35 players. This is when more players were pushed down a level.
At the D3 level there are plenty of colleges who will take as many players as will pay to attend the school. I know a kid who didn’t start at a small classification high school and was rostered for four years at a mediocre D3 program. For a few years his father puffed out his chest saying his kid played college ball while letting people assume it was the level they saw on tv.