Wait... Why can't VPNing be stopped? Sure, you're not going to be able to stop them all, but it seems quite feasible to block the vast majority of them.
First, for consumers to be able to find VPNs, they have to be publicly-available and advertised. What percentage of VPN services control 95% of market? A wild guess, but I would bet it isn't above 50, and very well might be below 10. Monitoring the top 100 services wouldn't require inordinate resources, in part because there's a wide range of companies interested in identifying and blocking VPN traffic for various reasons.
Second, if that fails, there are various techniques Netflix could use to attempt to detect VPN traffic using automated or semi-automated means. Netflix gets to see the requesting IP addresses for all their traffic. They even get to run run code on the client platforms, either because they (directly or indirectly) control the app used to access the content, or because they have the market share to put VPN-detection capabilities in the EME code for browsers. They can look at things like the number of different users connecting through the same IP address, ping/traceroute times, server and client-side IP address checks, etc., to heuristically detect VPN usage. None of those are going to be perfect, but Netflix benefits greatly by their size- they see a massive amount of traffic. If they use everything that is available to them, they can probably very accurately detect VPN usage.
Would they do it? Probably not, because Netflix is simply trying to show that they're doing due-diligence, and blocking using IP address blocks for known VPN services is sufficient for that. But, no, this isn't an impossible game of wack-a-mole for Netflix. Scale is on their side.