Actually, I've explained it on numerous occasions.
No, so far you just reiterate the advantages that more RAM brings, which is quite obvious. You're skipping a lot of the business decision making process.
Adding more RAM would result in a technological advantage over the competition (assuming they don't match it), while the actual "financial sacrifice" would be relatively small since RAM tends to be the cheapest performance component, and also the one which has the fastest and most extensive price reductions over time.
How is doubling the number of RAM chips the cheapest aspect when you haven't even considered the effect on the mainboard design? Relatively small does not mean it will be financially justifiable.
What makes you think the second company to launch won't try to be in the same ballpark of specifications just to placate developers and stop escalating the costs? At the end of the day, the majority of sales are from multiplatform titles. Platform parity has become a necessary mantra.
Shifty has already given a great list of areas where the budget could go instead.
It is also the number 1 bottleneck in consoles.
And at some point, the companies are going to have to draw a line as to what is financially viable against ROI.
You need to justify that $15 more of RAM (and that will filter down the chain as it has impacts on the size and design of the mainboard as well as the chassis and shipping weights for instance) will net you that many more buyers compared to the competition, and history suggests that mass market buyers quite frankly don't give a rat's ass if your console has even 8x more RAM.
If I'm not being clear enough, I'm talking about Wii's 64MB GDDR3 against Xbox360/PS3's 512MB. And to bring back the Xbox/PS2 example, again, double the RAM made no grand difference to sales. Your excuses that Microsoft was in a bad position, etc, actually help that side of the argument. Again, Dreamcast & PS2 - RAM was not the deciding factor. Gamecube vs Xbox - RAM did nothing when both consoles sold similar amounts. Your argument that there should be an increase in sales over competition due to RAM being in greater availability on a particular console is simply unfounded and baseless. There are more external factors to be considered instead of isolating the direct effects of greater RAM and making some errant correlation with sales to justify it.
Of course, devs will flock to where it makes sense (360/PS3 over Wii), but you cannot deny the hardware sales. Sony and Microsoft will be that much more cautious about creating loss-leaders.
No one is denying the boon to games that more RAM would bring, but you insist on reiterating that point rather than addressing the arguments thus far.
Also, if X720 has 8GB RAM the advantage cannot be optimized away on PS4 with 4GB of RAM. Graphics doesn't work that way, you actually have a very limited budget concerning RAM.
And devs will cater to multiplatform development. You trivialize too much the impact that 4GB more of features and assets would have to game development costs as well.
One of the systems will have a serious handicap and that's it.
And does that translate to sales? You are again ignoring the business side.