GA's second-biggest moat (behind being free) is its existing install base. It's already in place on approximately every website on the Internet. And no one wants to change analytics vendors, because that breaks history, which is a no-no. You want to know if your visits are up/down vs last month or last year, you literally cannot answer that unless you are using the same tool this years as you were last year.
By forcing a migration to a non-compatible version, Google loses that big advantage. If you're going to lose that anyways, then there's less of a reason to stick with GA. Now you're forcing people to gut-and-replace their existing data collection process, what incentive is there for people to stick with Google? One of the biggest reasons to stay with GA was just killed.
The timing is also exceptionally atrocious. Google is, again, forcing people to expend the effort to rebuild their analytics implementations from scratch. Meanwhile court rulings are coming down on a monthly basis enumerating different ways in which Google Analytics violates GDPR. A lot of companies might just brush off these rulings by simply doing nothing, but now they're being forced to actively invest time and effort into Google Analytics. A lot of them are going to be doing an evaluation against privacy-conscious competitors that simply would not have happened if Google wasn't kicking people of their existing platform.
The GA4 data model is also completely different from the GA Universal data model, which is an impediment to their fourth-biggest moat, the fact that every piece of BI software in the universe has a GA plug-in. Many of those plug-ins work despite not being actively maintained since 2015; the GA4 ecosystem is smaller and will take some time to recover.
(The third-biggest moat is Google Ads integration. I suspect that Google imagines this to be bigger than it really is in practice.)