A new hydrogen boiler costs 30k today. A new gas boiler is also not much cheaper then a heat pump and certainly more expensive to operate. Also if you can produce hydrogen on cost of fossil fuels, you could just convert some gas power plants to use hydrogen instead, as a backup. With heat pumps using enviromental heat as well, you end up using about as much hydrogen as when you burn it directly.

The key part is, you just can not just burn hydrogen in an old gas boiler. Even new ones can only burn mixtures. So installing new gas boilers instead of heat pumps is just wasting energy for no reason.