The message I've received from Fastmail is unequivocal, there is no chance whatever that Operamail addresses will be returning to use. The fact that not all Operamail account holders have received an email informing them of the withdrawal of the address is not because Fastmail is testing the water. Instead they are informing account holders in batches so that they don't have to deal with responses all at once.
As with another poster, I suspect there's a political agenda involved here as nothing else would seem to explain the unwillingness of the Chinese company that owns the Opera browser and domain to hand administration of Operamail addresses to another party. The email address itself has long since lost any association with the Opera browser so there's no way that Operamail can be accused of tainting the "brand".
Operamail was a good email address to have while it lasted, but a change is as good as a rest and I've decided to run with it.