That sounds like a much more graspable technical goal, but you would effectively be competing not with modern browsers but rather with the web as a whole, which

That sounds like a much more graspable technical goal, but you would effectively be competing not with modern browsers but rather with the web as a whole, which is a much more difficult social/political goal. If at the same time you could encourage widespread adoption of your HTML subset, then you may be onto something. This is essentially what Google did with AMP, their incentive being their caching network and apparent favoring in search results.