If some other practical carbon-free home power generation tech emerges (fusion?) the code will probably be amended to say "... or a 5 kw basement fusion reactor". That's how the building code has mostly evolved, by adding specific alternatives rather than trying to be entirely general.

Being specific has disadvantages, but the big advantage is that most decisions can be made by contractors reading the code instead of requiring an (expensive) engineering calculation. Engineering can be 1/3 of the cost of a remodel as it is. Increasing the engineering cost gives a big advantage to tract home builders who can amortize the engineering across a subdivision, which results in suburban malaise.

For instance, current energy codes require a certain R-value of insulation in walls, so that any contractor can go to Home Depot and buy the insulation marked with that R value and expect it to pass inspection. If the code said "Homes may only lose X BTU/year through all surfaces" it'd require a lot of engineer time per house to work it all out.

It's great that solar has evolved from being so expensive that it could only be installed in the most carefully optimized system, to something where you can just slap a few panels on a roof and expect reasonable results.