Although I’ve worked with hydronic heating for four decades, and designed systems around just about every possible heat source, I would be hard pressed to predict what might be available as hydronic heat sources 25 years from now.
Fifty years from now, I doubt that I will be predicting anything, and yet, hydronic heating, in some form, will hopefully still exist. If comfort, efficiency and lasting value, rather than first cost, become established as the market “drivers,” hydronics might even be the dominant method of heat delivery. Imagine that.