I don't think it will all be hydronic floors. The cost is up there for the installation and until air conditioning is done regularly without ductwork(such as mini-split setups) there will be ductwork in a house and the additional price of having a furnace versus just an air handler is very small. Not to mention the convenience of quicker temperature changes that come with forced air versus waiting for the water to warm up in the tubes and the work its way up through the flooring and then eventually into the bed, couch, and other furniture to where the room feels warm. Forced air heating is cheap and easy to setup and if the ductwork is insulated and air sealed when its installed, which would be less effort and cost than running hydronic, then there is little efficiency loss to forced air. Furnaces are at 96.5% efficiency and electric usage is down to 166kwh heating for a year, for example with a Carrier 59TP5A040E14(40k BTU/hr furnace). The advantages are slim and the cost differences are large and a 50k boiler uses 275kwh of electricity for a 90% and 500+kwh for something 95% or higher. Not to mention only 5 companies make a water based, intermittent ignition, natural gas, 95%+ efficiency unit. Finding a natural gas furnace with a variable speed blower and 95%+ is easy and less expensive.

I think its mostly up to what the consumer wants and what builders are willing to provide, right now both are pointing at forced air, mostly for the cost/benefit reasons. I'm personally not seeing the benefit of going water but for those who go that route I'm sure it's not too hard to find a builder who specializes in it. Most houses used to use baseboard or large radiator heaters and many people see that as a thing of the past, might be quite a barrier to their reintroduction, along with the fear of water damage if one of these gets punctured.

If you want to do low temperature heating, and solar is low temperature heating, you need very large area radiators.

Hydronic floors are very large area radiators, and when they are made correctly they are silent.

Forced air does not work well with lower feed temperatures, unless the air handler and ducting system and blower are all designed specifically for low temperature heating. This would mean a much larger than typical surface area in the air handler, duct work that is capable of quietly handling continuous air circulation, and a blower that is very high efficiency (ECM) and is able to adjust it's speed over a wide range, because it will need to run just about constantly.

-AC_Hacker