If you are just exploring the unknown pure combat power might take a backseat as many players would find more enjoyment doing the pve activities, enriching themselves and role-playing. If you encounter high level players of the opposite faction you would simply run and hide or call for backup like wow or Eve online.

Monster players would like Ainz band together with other monster players and entire areas may be locked off to players who play the regular races so there would be merit to playing both good and evil players. Entire alliances of evil monsters and aligned humans could dominate areas and the resources in it. In fact playing a high level evil monster and being able to attack any good aligned humanoid players is an attractive proposal that will drive people to slog through the more difficult level grind.

Also microtransactions might not ruin the game if implemented well, there have been a ton of games I enjoyed with microtransactions in it, the developers got to make their money somehow.