not playing your best players will make them drop moral
and doubt experience count in the game tbh , i think the engine just look to the age .. this is why all match a player played in a season is erased by new season
meanwhile, if you play first team, you make risk some injuries too .. which can carry on to next season
.. what i do is often play first team against teams that don't injury players and give the reserve a chance facing aggressive sides that cause injuries