What are the incentives for more paying customers? Wikimedia Foundation, being a non-profit, has a lot less incentives to chase money at the expense of product quality.

The for profit parts are meant to chase money, and higher revenues justify higher management salaries ("I am running a bigger organisation so I deserve to be paid more"). This is widely accepted as being a motive for misaligned incentives in for profit businesses (with regard to takeovers) too.