I still don't really understand the apparent reticence to get rid of the Premium account tax on mainland and sell mainland to any (payment-registered) Resident?

Without the Premium tax mainland would cost 1.3 US cents per metre (and 1.5 if you have to pay VAT) making it the best sqm value in SL. The fact that it comes with neighbours and has terra-forming restrictions would be a fair compromise at the price-point.

Some Premium account holders (other than me) might complain but could be compensated with a much larger Premium land bonus with their account - maybe up to 4096m instead of 512m?

If, as M says, the Premium subscription is irrelevant to their business model then get rid of it, give people more land choices and revitalise the mainland - instead of just slapping price-hikes and reduced-spec's on their existing customers!