I do wonder how many use cases actually need more then 32GB when you have a low latency NVMe flash with 5+ GB/sec of bandwidth and relatively low latencies. Especially with the special magic sauce that I've seen mentioned related to hardware acceleration for either compressing memory or maybe it was compressing swap.
In any case, I'm not expecting the top of the line for the next releases. Step 1 is low end (mba, mbp13", and mini). Step 2 is mid range, rumored to be a MBP 14.1" and 16 in first half of 2021". After that presumably the mac pro desktop and Imac's "within 2 years". Maybe step #3 will be a dual socket version of step #2 with of course double the ram.
Also from the ASIC yield view it allows for some interesting binning of chips. Let's say the M2 has 32MB on chip plus an off chip memory controller. They could use the ones that pass in the high end, then once that fail a memory test as 16MB on a laptop, etc. Part of keeping ASIC cost down is building the exact same thing and binning the chips in to devices based on yield.
Maybe editing 4K (or in the future, 8K) video might need more?
My brother does a lot of complex thermal airflow simulation, and his workstation has 192GB of RAM, but that is an extreme use case.
And it can handle 8K for 1-2 streams and starts to lag at 4+.
Apple in making the M1 Chip is using some of the Amiga IP circa 1985 that speed up the system where the CPU and GPU etc. share memory. Amiga is shattered into different companies, but if they didn't go out of business they would have made a M1 type chip for their Amiga brand.
It would be sort of interesting for Intel/AMD to do something like this where they have 16GB on the CPU and the OS has the knowledge to see it differently from external RAM.
Apple is going to have to do this for their real "Pro" side devices as getting 1TB on the Mx will be a non-starter. I would expect to see the M2 (or whatever) with a large amount of the basic on chip RAM and then an external RAM bus also.