Right, I mean even a fast SSD has an order of magnitude less throughput, and 2-3 orders of magnitude higher latency from RAM. No dispute there. If you are doing random access across 16GB of data and your machine only has 8GB of physical RAM, you're in the pain zone.
OTOH, if you are using multiple RAM-heavy apps that aren't actively hammering that RAM (e.g. an instance of Photoshop that is using 10GB but is just idling or whatever) then MacOS and their stupid fast SSDs handle that pretty seamlessly.
Most use cases are probably somewhere in the middle.
