Generally, trying to directly convert a free service to a subscription service can be much harder than starting out as a subscription service. Just look at all the resentful conspiracies about Facebook planning to charge money that would go viral back in the day.

Users don't like a contract radically changing from under them, and shifting from free to paid is breaking a contract in an immediately understandable way.

No one was forced to buy the plan nor was the free Facebook going to go away. You just would have had the option to pay to not have targeted ads. And that was vetoed by the EU, the very thing many here claim they'd like to do.