By making a great product, they attract users. Usually the main argument against software like this is vendor lock-in. But by using open formats, there's always an escape hatch. If they decided to squeeze their users, I suspect a decent open source alternative would pop up very quickly. In the meantime, casual open source alternatives are very unlikely to catch up in terms of quality and features, because they don't have a funded team working on them full time.
On the flipside, Obsidian is incentivized to keep their customers happy, because their moat isn't so large as to allow complete complacency.
Overall some of the best aligned incentives I've seen. Though it does make me sad because I feel like we almost never see this good of balance in open source software.