There's a flaw in your thinking.

Let's look at the read speeds of two of them...

PS2 had a memory bus bandwidth of 3.2 GB/s. PS4 has a memory bus bandwidth of 176 GB/s. That means that, in two generations, it's increased by x55, or a typical increase per generation of x7.5 (keep in mind, these are theoretical peak bandwidths, so it's not accounting for time required to search for data). So memory read speeds are limiting the benefit of the growth...

... but the bigger issue lies in the reading of data INTO memory. For discs... PS2 could read data in at a rate of 5.3 MB/s, PS3 could read data in at a rate of 9 MB/s, and PS4 can read data in at a rate of 27 MB/s. So it would take about 5 minutes to read PS4 blu-ray data into the PS4's memory to fill it up (compared with about a minute for PS3 and 6 seconds for PS2). Following this trend, even with a strong improvement in read speeds on the next disc format, it would end up taking of the order of 20 minutes to load it in.

Of course, a solution to this is to use hard disk data... but that only delays the issue. Even the fastest regular HDDs will only manage of the order of 100 MB/s, and while an SSD would improve on this, SSDs have an issue with frequent access. And even if we suppose the fastest of the SSDs were used, we're still talking less than 1 GB/s. So for 128 GB of RAM (not sure why you said 131 GB), it would still take 2 minutes to fill the RAM from the SSD.

The point I'm making, here, is that it's unlikely that so much memory would be used. Far more likely is that they'll save costs along the way by choosing a more manageable amount of RAM - maybe 16 or 32 GB, and instead spend the extra money on the cache, which is more likely to be beneficial following the trends at hand, or perhaps dedicated sections of RAM for special tasks that can be optimised in some way.

Keep in mind, this is likely to also apply to PCs, except the very top-end ones (as in, ones being used for professional/academic purposes). So it's possible that the RAM in the PS5 will be similar to that found in typical gaming PCs of the time. But it's unlikely that they'll go as high as 128 GB so soon, barring some significant breakthrough in technology (that would need to go beyond the trends given by Moore's Law, since that doesn't predict doubling every two years for speeds).

Also keep in mind that much of what gets loaded into RAM needs to essentially be hand-crafted at some point - we're talking meshes/textures/etc. There's already a problem with the cost of graphical designers in videogames, and increasing it by a factor of 16 again would basically kill most video game companies. Since very few would ever try to use that much, putting it in there would be a waste. It's likely that new techniques will be developed to try to cut the costs in this department... but such techniques would likely also cut amount of memory required (by being able to generate the textures/meshes/etc on the fly, avoiding the need to store them in memory).

To summarise, while the "optimal" amount of RAM that could be put into a system for a specific cost should roughly double every 2 years, other factors mean that RAM costs should instead begin to decrease, rather than keeping it constant with rapidly-increasing RAM size, as Moore's Law doesn't apply the same to every facet of technology.

I am calling it now: PS5 will have between 16 and 64GB of RAM. Most likely scenario is 32GB in my eyes. Anyone want to bet?

theres no such thing as 131 Gb of ram...

128Gb could be possible, but games would have to be in the ballpark of the 500Gb+++ in file size, if games do get that big, ram will follow. unless of course, devs would want to run the game entire game from ram, putting the loading screen in a distant memory. (human memory that is...)

Indeed, there are still very few games that require as much as 8GB or RAM, to think that it will become the norm to require 15-16 times as much in the span of only 5-6 years is rather crazy.

Besides; there's more to Moore's Law than simply doubling on units etc, most might have noticed that the number of cores in CPU's has not doubled with each  generation, nor has the onboard video memory on GPU's doubled either. The architecture itself becomes more complex over time, allowing for more circuits and performance on each individual unit rather than requiring a constant and relentless need for adding more hardware parts and size (as in the RAM case). RAM is no different, each GB og memory is becoming more and more adept at its job, where my first build some 7 years ago had 2GB of RAM that ran at around 833MHz and was considered top tier, 1600MHz and up to 2300MHz has now become more common and the bandwidth is literally off the charts compared to old memory and the actual amount of memory in raw figures hasn't really increased that much on average over the course of those 7 years (my current system has 8GB of RAM). Heck; there are some massive titles, Skyrim being the most famous, that needed its own patch in order to use more than a given amount of RAM for rendering purposes (it was locked at 3GB expenditure at launch).

The GPU I'm getting after new year's is immensely more powerful than the one I currently have, despite the onboard memory being "only" twice as big, 4GB against 2GB. There is more to the puzzle than GB's when it comes to hardware and there is not a single title on the far, far horizon that will require even half of those proposed 128GB's of RAM. Infact; I dare say that by the time software has become that demanding and taxing, I don't even think rendering processes will be run the same any longer (more use of cloud processing and other solutions).

I suspect that rather than growing to 128GB, we will instead have 16GB of stacked RAM on the die, with 5 to 10X bandwidth improvement (1~2 TB/s bandwidth).

That would make more sense, a larger cache or memory also requires much faster transfer to handle large batches of information. If I were to guess a figure; I'd say that it is highly unlikely that an eventual PS5 will have more than 24GB of RAM, with 32 being the at the very ceiling of ridiculousness.

Gotta have some power to it. 4K graphics need some power behind it.

RAM has nothing to do with power. Power comes from the CPU and specially the GPU choosed.

RAM only tells you how much data can the console work at the same time and how fast they can access it.

I'm waiting for 1TB/RAM (:

That's a good point; it's NOT the AMOUNT of RAM, but the SPEED of it will be the bottleneck. PS4 is fine this generation but XB1 is in big trouble. The generation will have to increase the RAM speed (bandwidth) by 4-8 times, a similar improvement as the GPU. Also the CPU needs to quadruple its speed at least.

4x CPU speed + 4-8x GPU speed + 2-4x more RAM + 4x faster RAM

That's the future generation console (in 6 years)

- The CPU technology is already here. An i5 would cut it. It just needs to be more energy efficient and integrated in an APU package.

- The GPu technology will be here in a couple of years, but again it needs to be "mobile" and "integrated" in the APU.

- The RAM amount and speed are the slowest improving parts and they will be "cutting edge" with the consoles.