I don't think that's baseball year round making them less athletic, I think it's that less athletic players are more likely to be able and willing to play baseball year round, and that allows them to stay ahead of more athletic players and possibly pushes more athletic players to other sports. Our friends that have done travel ball for U8 coach pitch have two or three good athletes on the team, the rest are just there because they are affluent and have been worked with a lot, both by their dad's and by private coaches. Some of them are probably going to get pushed out each year as other players athleticism overcomes the advantages of more reps that the less athletic players have. But not all of them will and on average, they will be less athletic, if more skilled, than the equivalent teams when we were growing up.
I firmly believe that baseball year round does make players less athletic because baseball is not athletic by its nature. If the year round low calorie burner was to play 4 month of basketball or soccer, his quick twitch muscles would evolve to another level and that would translate into a better baseball player.