I have no idea if Solar City has 20 year contracts with the various utilities to back up their arrangements with homeowners, seems doubtful and that would be a major weakness of the situation. Plus I think it is open to possible changes by the States' PUC over time. Twenty years is a long time in regulatory terms.
Overall, you have a homeowner, the utility, the PUC regulators, now add in someone leasing your system to you with their own agenda. Seems like the homeowner is likely to be low man on the totem pole in all that.
Elon Musk is no dummy, I hope he can deliver on the batteries. My guess is that he is betting on a regulatory direction that works in his favor for the storage/battery market while improving the production efficiency of the batteries to lower the costs at the same time. You have to figure he is planning his chess moves 5-10 years out.