So: one option is to introduce a third petrol: so compelling larger retailers to sell E5 as well as E10 - but only for about 18 months. I think that's hopeless - too few drivers will buy it.
Another is that the 'Super' grade should stay at E5. Only 5% of fuel sold is 'super' (and in my limited experience you cannot buy it in the few remaining garages in rural areas). So we'd pay an extra 10p per litre for a less worse option. The advantage is that it would be more likely to remain on sale indefinitely. So that's my favoured option.
What are yours?
I cannot believe compulsory E5 at premium price but not premium performance will sell more than a couple of percent of fuel - so it will wither and die from lack of support.