I would imagine Valve making more money from Steam now than from their own games. Remember they sell a lot of third-party stuff there and take their cut on those sales.

They make lots of money on Steam. They take a healthy cut of everything sold on the platform.

As a gateway to all their games, is Steam critical to the company?

It's the difference between being a distributor vs being a creator.

Steam is the most critical part of the company for their future.