There are enormous piles of money looming around every corner seeking a return on investment. If you have users that are enjoying a service, one of those piles of money can buy out the owner, double the price, implement ads, and sell all the private data. The bet they are making is it will take longer for the userbase to quit than it will take to make back their investment.

Every popular / beloved service is a target for these giant piles of cash. The fact that lots of people like it is de facto proof that it's underpriced, or over-resourced, or coddles its users with too much content. According to the finance industry, a stable business relationship should have the userbase reluctantly concluding that they have no other option, gritting their teeth and opening their wallet - and that's the sort of maximally profitable entity that a giant pile of cash will leave alone, letting it just exist, as a business.