mere mortals at what point is there so much stuff in memory that you need a couple hours of hold up time just to flush it out to SSD

If your buying these machines with multiple TB of RAM, then buying flash arrays that can drive multiple GB/sec of IO bandwidth shouldn't be a problem either. At say 8GB/sec write bandwidth flushing 1TB is a little over two minutes. Although, why you have that much "dirty" data in RAM might be another question. A database machine using that amount of RAM is going to care more about the IOP rates of the disk, so that its flushing updates to disk at the same rates they are arriving. Meaning that the RAM won't need to be flushed to disk if the machine/power/whatever fails. Disk arrays with >1M IOP/s have been around for over a decade, and given a SAN can be wired together to increase aggregate performance.