My $0.02 and I am in the solar industry for full disclosure.
If there truly was a level playing field, solar and renewables would be winning hands down. Grid parity was reached 5 years ago nationally. Depending on sources of information, fossil fuels and energy derived from them receive at the very low end $1.5 trillion in subsidies annually to a high end of $5 trillion. That doesn't factor in the human cost of death and injury of men and women in uniform protecting fossil fuel supply, or death and injury from localized pollution and environmental damage.
When you factor those true costs in, solar and renewables win hands down. However, because of the non-level playing field, rebates and subsidies are in place to encourage adoption. One person's subsidy is another person's investment. If NASA had not paid extortionate amounts for the original microchips, we would not be on this forum now.
California making solar mandatory for new construction is a response to the obvious. We are as a species moving towards renewables. It makes sense for economic, employment, manufacturing cost, environmental and national security reasons. When the World's 5th largest economy does this, other States and countries will follow. It is an inevitable journey to renewables no matter what we think or how much we really "dig" coal.
So what is the impact? Well it will increase CA's solar adoption at an accelerated pace. Currently at around 7.6% adoption, that will quickly hit 10% which history shows is a number that spurs mass adoption. For the homeowners "forced" to buy solar with a new home, they pay up front to save many fold over the years from reduced or eliminated utility bills. For existing homeowners, they will need to make a decision whether or not to bring their properties in line with new construction to compete on resale price. It is likely that many will, especially as rebates and net metering mean there is no out of pocket to do so (in most cases). Fannie Mae allows a full appraisal value of an owned (but not leased) solar system.
However! "Mandatory" and "Incentive" do not work together - no one pays us to wear seatbelts. I think this measure - especially as it moves across the US - will lead to an accelerated decrease in the Federal tax credit - currently 30%. Also as benchmarks are reached in terms of adoption, utilities can renegotiate net-metering rates to pay less for the excess produced by a solar system.
In short then, the conditions to go solar now are certainly as good as they are ever going to be. It will be "good" in future, but I believe increasingly not as good as now.
In California and I believe every State, there are laws in place regarding the efficiency of new construction - insulation, double glazed windows etc. They are paid for up front by the homeowner who then benefits from reduced utility bills. This mandate for solar on new construction is no different to that, other than it is a true investment in owning your own power rather than renting it from a "public" utility.
The main down side of this increase in renewables is the "duck curve". This is the upsurge in power required when solar is not operating after dark. In order to meet that demand the utilities have to keep powerplants running to ramp them up. They cannot just switch them on, so they bear a cost in over-production. The solution to this is in storage. That is the weak link in renewables now. True daily use batteries for economic home use are a year or two away from mainstream. For utility scale, even further. Investment (subsidy) would accelerate this technology and manufacturing to make storage economical. It's happening in the private sector, but a NASA type investment as happened in computing would be awesome.
As far as "Govt telling us what to do", well I've been in LA for 25 years. Anyone that has been here for any length of time will tell you about the massive positive impact of clean air legislation. Sometimes Govt involvement makes sense and I think we'll look back on the CA solar mandate as one of those times. The fact that major manufacturers like Volvo are eliminating fossil fuel powered vehicles is a great indication also of where CA and the rest of the World is heading in that regard.