The Tesla plan is to decentralize the grid load in this way, back-feeding from EVs, powerwalls, megapacks etc...

Works great for off/micro-grid, but the issue of scale (and getting it to pay for itself) with the legacy grid is that the legacy grids can't keep up because they weren't built for bi-directional flows.

At my past employer we had several utility-scale solar arrays (stupid for a bunch of reasons but I digress) that sat unused for years because of the backlog to get them connected up to the utility

It doesn't make sense for building construction to chase this imo, its a separation of concerns... If you need back-up FROM the grid in a reliability sense that's one thing, but otherwise megapack peaker plants are the soft-fork approach with the legacy grid that will leave it uneconomical for general purpose buildings to partake.