(a) consent to personalised ads
(b) subscribe
(c) do nothing
Will Meta block people from using its websites if they refuse to consent to personalised ads, i.e., option (c). That would seem quite stupid. Meta would lose the traffic.
If millions of people consent, i.e., choose option (a), it defeats the purpose of GDPR. Meta scores a victory against privacy. They may even succeed in complying before the ban comes into effect.
If millions of people choose (c), i.e., to retain their rights under GDPR, then Meta is fscked.
EU is forcing the issue, but Meta will only present this choice of options to people over 18. Quite a large carveout.
Users in the EU should choose (c) and call Meta's bluff. There is no sensible reason that Meta would block users who do not consent to personalised ads.
If a user wants personalised ads, they can opt-in. Users who find personalised ads useful, such as the lone outlier commenter at the top of this thread, can opt-in.