it's not like it's the only league in the world to have a reserve team in it. Don't see too much downside to be honest.
Which leagues are those then?
Bundesliga and La Liga have 2nd/reserve teams in their system. Until the advent of a national 3rd league, reserve teams could play in the 2nd Bundesliga. I think reserve teams can play in the 2nd tier in Spain. I think in both instances the reserve team and 1st team can't be in the same league as each other, so can't be promoted if 1st team is in league above or get dropped a division if 1st team get relegated to the same level.