Buffering is already done as those devices already have RAM used to buffer. That said in new computers ODD's are basically obsolete unless you are making a HTPC.But the point is that it would help out with buffering and lowering latency along with massively improving bandwidth once the data is loaded.
Not true when looking at a SAN solution. Using Optane in a VMware vSAN as your write cache can increase your DB write performance by 25%. Reason is the high IO at low QD. Do note that in an all flash VMware vSAN the cache drive is only write cache and all reads are done from the NAND storage layer.There's literally no point in using Optane with existing NAND flash, the benefits are minimal.
The OS and applications are growing all the time. My work laptop has a 256GB NVMe SSD and I have 110GB free and that is only with the applications I use all the time for work. As it ages and more service packs are added the storage will continue to increase. Also due to the cost of drives there is little reason to go with a smaller OS drive anymore. For example the cheapest 256GB NVMe drive on pcpartpicker runs $20 whereas the 512GB runs $29. Why would you limit yourself to only 256GB when you can double your storage for less than a 50% increase in price?Why don't you want smaller than a 256 GB OS Drive?