Haven't read every line of this whole thread, but it seems to me there are some missed points and clarification needed. Referring to a concept from early in the thread, I am not aware of any fundamental issues that prevent a solution. But there are very real implementation issues, most of which have theoretical solutions not all of which are fully implemented as renewables exist today.
Issue 1. As the solar system in my home is configured (and probably most grid tied systems in the world), when there is a fault on the incoming line the system disconnects from the grid. This is done for the safety of grid workers as they repair the fault. It is bad for linemen when they find a "disconnected" line energized by attached renewable sources. This doesn't cause any really serious problems when renewables constitute a small fraction of total generation. But when that proportion becomes significant there are various issues. When the fault is due to one of the remaining large singular generation sources failing the effect is magnified by the loss of all the attached renewable. And there are startup problems. There are lots of ways to solve this problem, but to my knowledge there isn't much effort yet on implementation. As mentioned previously, no one wants to pay for it.
Issue 2. DC grids are not automatically stable. The issues are different, again all solvable AFAIK. But there has been next to no investment in DC grids with more than a few dozen sources and sinks. Even fewer where operations are split over a huge number of owners and organizations.
Issue 3. Something I am surprised that no one in this forum has brought up is filtering for the noise caused by power lines. I have several pieces of equipment that have notch filters for power line frequency. If power line frequency is allowed to drift widely from current norms these filters will be noticeably less effective. There are other effects from allowing large drifts in frequency, mostly involving changes in line loss, reactive power and core loss in equipment that is often carefully optimized for one frequency. Equipment from a 50 Hz network usually works on 60 Hz and visa versa, but not as well. Clocks are not the only reason to keep power line frequency relatively constant.