I'd say they can go with 32GB

What for?

It's already pretty damn good. 128GB would be excessive to me..imagine how long the loading times would be..

32GB..

My take is 16 - 24GB.

Andto all those talking about 8GB phones.......... not all Ram is made equal.

32GB total system RAM. I'm sure ;)

RAM is an expensive component. I highly doubt it will be more than is needed to fulfill their overall goals. Don't be expecting something crazy like 64GB or worse, 128GB. It would be a waste and likely go underutilized. Better to put that money into processor or other components. Specs are not always about the most, but rather the right balance for the times and the goals of the device.

Well.... in their defense... PS4 came out at the end of 2013. In that year we had the HTC One, Samsung Galaxy S4, iPhone 5/s, and Nexus 5.

HTC One had 2GB of ram at $600usd
iPhone 5 had 1GB of ram at $649usd
Galaxy S4 had 2GB of ram at $650usd
Nexus5 had 2GB of ram at $400usd.

Yet PS4 releases same year with 8GB GDDR5 + 256MB DDR3. 4x the ram of the beefiest flagship and at the cost of the cheapest one. 
My OnePlus 5t has 6GB of ram and I got it for $450 (student discount pricing) last year. The 8gb model was an extra $60 i believe. It's reasonable to think Sony can do much better in terms of GB/$.

There's a lot that also goes into those phones that we forget to count like the constantly changing assembly line, faster pace R&D, water-proofing, certifications, smaller form factor, royalties and anything else that's more costly due to yearly releases.

EDITED

Here's a wild idea: build the PS5 with expandable memory. I know that goes against the console edict of "fixed hardware/closed box", but we've already seen successful mid-cycle hardware upgrades (XB1X, Pro) that co-exist seamlessly (in terms of games) with their lower-powered siblings.

So why not build the PS5 with on-board 16Gb RAM, and then have a user-upgradable DIMM slot so that down the road when Sony and game devs need/want to support it, users can expand the memory to 32Gb. Newer games could take advantage of the doubled RAM for all sorts of visual and performance improvements, while non-upgraded consoles would maybe experience longer load times from HD, less-detailed texture maps, etc.

And speaking of the HD, I'd much rather Sony invest in a high-performance interface like M.2 to improve data throughput than try to stuff 32Gb of RAM into the console at launch.

Not only is it not a possibility, but it is not even a good idea. Why would you need 128GB of RAM for gaming purposes? What are you going to run there? Galaxy simulations and quantum mechanics simulations? This amount of RAM is useless, unless PS5 supports 16K resolutions...