I think the utilities are moving very early with their flat rate charges but I don't think they're wrong in the long term that a flat rate will be required to fund the grid in the future. I'm thinking about the point where a large majority of customers have sufficient solar generation to cover their entire energy usage for the day on average, those people still need generation or storage of power during the night when solar doesn't work so somewhere they'll need to continue paying for power generation or storage during the night. This is probably doable with time based rates instead but we'll have to see and even then we'll probably need some flat rate to account for people with local storage because they also exist as a cost to service.
So as I mentioned in my previous comment, public utilities commissions across the country have all run their own independent studies of the value of bringing solar on the grid measured against its costs and generally found it to be a net positive rather than a negative. Those studies encompass things that you're talking about such time base rates, cost of mobilizing peaking and base load production, efficiencies from consuming power on site instead of having to send it through the transmission and distribution system, etc
I'm not talking about now. I thought I made that clear enough but I'm talking about in the future with extremely high levels of solar self sufficient customers. All of those need power during the night so absent huge grid storage you'll need nuclear or hydro to provide reliable night time power all of which costs money.
I'm all for massive solar investments it's a great path forward but there are issues we'll have to address.
