Gas boilers are still going to be around for quite a while yet.
I've a feeling the gas network might well get changed to methane at some point in the future, Scotland are on about doing just that.
It'll be no bigger a deal for a modern boiler than changing the gas jet and venturi.
It was before my time, but we've done this in the past when we changed from 'coal gas', I can't see it being a big problem unless you run something truly ancient.
Even with oil, you can change to kerosene, which is more expensive but you do get better efficiency out of it due to its higher calorific value and cleaner burning. In my opinion we should be running diesel cars on kerosene, but that's a different story
Biomass /wood chippings is a monstrous waist of resources. Currently the UK does not produce any pellets itself. It all comes in from Canada via Ireland and isn't cheap and is delivered while they're on their rounds or on a rota. Not as straightforward as just asking for some.