I’m surprised it’s not per-seat or per-user. Not like the dev is getting more money if the user re-installs the game. Also not a fan of it being monthly. I get why you would charge twice if the user installs it twice since you may not be able to track concurrent installs without DRM, but that should only apply if you choose a per-install licence. Per-install also opens you up to malicious users installing/uninstalling to make you pay.
There should be a per-seat/per-user perpetual price if the dev never updates the Unity engine itself. I get charging per-seat/per-user monthly if they devs are pulling in new versions, but that should stop if you cease updating.