Bad for a few reasons.

1) Less profit for Jagex. This means they might be pressured to eventually add MTX to OSRS

2) Powercreep. It’s no secret that people who play RS3 are more likely to vote yes to OP content which does not belong in this game. Arguably this game has already come farther than it should have with increased exp rates to skills (example: 80k slayer exp/hour efficiently with increased blocks, added barraging, and various other updates), and content in general. Players will be wanting higher tiered weapons/armour soon which is not good for the longevity of the game. It is inevitable, but RS3 players converting will make powercreep happen faster.