Upgrading Ram is pretty easy and cheap right now on PC. I feel like Sony, MS and Nintendo should have given us the option to upgrade the Ram. Even on PS4 and Xbox 1 it could improve in performance a lot with 16gb for a lot of games. DDR4 comes also very close in performance to GDDR5

Nope, that would cause more optmisation problems.

The N64 and saturn had no optimisation issues and they supported Ram expansions.

No. RAM isn't a big issue. It's other things.

the n64 needed it because rare couldnt get DK64 running without^^

Bad idea because games will be optimized for the upgraded hardware thus making the base model irrelevant and requiring upgrades to play. Or games will require more dev time to optimize both hardware.

N64 is different because most games that required it came with the upgrade when you brought the game (i think)

Nope. If you want to upgrade things to get better performance buy a PC. Secondly why would you want to upgrade the ram? The current consoles arent even strong enough to truly utilize the full 8 GB as is. As far as DDR4 goes it would be impossible to upgrade from DDR3 to DDR4. The Chipsets that the consoles use dont support it and it wouldnt fit in the same slot.

Memory capacity isn't much of a bottleneck so been able to upgrade it doesn't amount to a whole lot of performance gains ...

You start with a cheap upgrade, and before you know it, you have a few devs who couldn't possibly make their game work without the extra few gigs. I doubt the pros outweigh the cons.

As far as I'm concerned, the expansion pack limited some games. Majora's Mask was only playable with it iirc, and that's not very good... especially these days.