Most vaccines would have been developed around the Alpha variant and I agree the Delta variant is less deadly.
As you say, there is a tendency for future variants to be more benign
But the origin of covid is unclear and it's possible that a variant could be very contagious and more deadly as happened with the Spanish flu that killed few in the first year but killed large numbers I'm the second year.
You idea of 1270 deaths in the UK is is not useful because it doesn't account for the 50 odd million who were vaccinated and less than 10 million mostly younger people who weren't vaxxed.
