Because leadership and experience is important to have around new players as well, look at CG this split or GGS a split ago and you can see how having a very fresh squad can be difficult.
Can agree with this concept but not in how it's put into practice. I think the academy teams shouldn't be recycling players that lose their spot in LCS starting rosters, at least not so often.
Have veterans to anchor new players on starting LCS/LEC/etc teams because they are players trusted to actually play at and think at that level, but
1) Don't want a player who isn't viewed as good enough for a starting roster to be the one influencing the new academy players,
and
2) The goal of academy should be raising that talent, and the experience they get presumably scrimming and doing other practice and such with the main roster should be what does that. They don't need a veteran player from the LCS on their team.