With the assets already in Spiral Knights they can create a Vampire Survivors mode.

The permanent upgrades outside of the game loop is gear, and the in-game bonuses can be anything from powerups to just stat increases.

They already have the engine to support many enemies on the screen, and 4 player co-op. This could be a high return on investment business decision. It can even be a separate game called: Spiral Knights: Survivor, or something.

This would allow the devs to reuse assets and tech, which is cheaper than creating brand new stuff, while also not needing to worry about character progression, and tying everything in with the old. Ofc, they can if they want, I'm just saying.

Think about it. LoL has proved that a 3D variant of Survivor games are in-demand.