I think some of this can be guessed from the SoC codenames

https://en.wikipedia.org/wiki/List_of_Apple_codenames

M1 Max is Jade C-Die => 64GB

M1 Ultra is Jade 2C-Die => 128GB

There is a still unreleased SoC called Jade 4C-Die =>256GB

So I think that's the most we'll see this generation, unless they somehow add (much slower) slotted RAM

If they were to double the max RAM on M2 Pro/Max (Rhodes Chop / Rhodes 1C), which doesn't seem unreasonable, that would mean 512GB RAM on the 4C-Die version, which would be enough for _most_ Mac Pro users.

Perhaps Apple is thinking that anyone who needs more than half a Terabyte of RAM should just offload the work to some other computer somewhere else for the time being.

I do think it's a shame that in some ways the absolute high-end will be worse than before, but I also wonder how many 1.5TB Mac Pros they actually sold.