I think that is the risk they run, the community has been formed over the years since CClassic, if now half the community is leaving (due to this remarkable change), it will not return in the short term because they will already be with another engine and do not want to buy a new one (C3) in the event that the subscription is deleted.
If I go to another engine and learn it and have paid for it, I probably would not go back to C3 even if Scirra changed the model to single payment.
I understand that they want to be novelty but I guess this is one of the risks.