Merging NA and Eu for PvP
One side would lag. Try playing on an EU server while you live in NA, or vice-versa. Your ping would put you at a disadvantage.
There is enough players just everyone stuck in 2 or 3 division, there is no progression having 50% win ratio.
Probaly division4 players can be matched only with own and division 3 and 5. In time there will be more division3 players, but most of soloquers will stuck there, so same issue will repeat with division5 matching.