Why is it supposed to be an investment? If it's sufficiently cheap, it becomes something that you buy because it's useful to you, not because the government is promising you subsidies for life.
Also, the other driving force here is the cost of grid-delivered energy. California is sure working hard on incentivizing solar + storage that way...
I think the person who is selling "cheap" solar will decide it isn't worth it to give you something convenient without additional cost, especially if they are losing money.
They will exit the market all together cause there are other ways to spend money than give away cheap solar.
Also, the other driving force here is the cost of grid-delivered energy. California is sure working hard on incentivizing solar + storage that way...