Their incentive is to keep the browsers good enough to not lose market share. Other than that, the incentive is to either close the web down, or to make the experience as shitty as possible without leading users to switch away, so they can steer users towards the more closed-down native apps.

Unfortunately, companies have an incentive to put us into walled gardens, so the only company that actually cares about the web is the company whose only business model is selling a browser.