You paid for the single player experience. This is an entire new game mode which might as well have been a different game. You should respect the developer’s time.
With how crazy high game prices are nowadays, I actually respect Mike for keeping the Steam price the same as the Itch price.
If he had made this into a DLC that cost $3 (the price of the game on steam), no one would have complained.
Also the price we are talking about here is on the level of spare change. This game could have easily been priced at 6-10$ on Steam with the release of multiplayer.