That... doesn't work. The games which were released for the N64 were all designed to run within the limits of the hardware. Adding more memory later wouldn't give existing games higher resolution textures.
Plus, adding more texture memory would have meant respinning the RCP silicon. That would have been a significant expense for marginal returns.
Besides, Nintendo had already sold a memory upgrade in the form of the Expansion Pak. A second memory upgrade which couldn't even be installed into existing consoles would have been a very difficult sell, and could have soured customers on their future consoles. ("Why buy a Game Cube when they'll just release a Game Cube Plus next year?")
It only works today because devs can update games post release and there is enough speed to use abstracted libraries rather than designing a game to the letter of the spec sheet
DK64 required the expansion pak due to a bug (it was intended to work in either 4MB or 8MB modes and they couldn't fix a crashing bug in the 4MB version) and then Rare spent an obscene amount of money shipping an expansion pak with every copy of the game. That was probably responsible for a large fraction of the units shipped, with most of the rest accounted for by Zelda (which did not include the pak even though it was required).
The lead artist claims they’d always been planning to make use of it. There’s compelling evidence of a memory leak or similar issue since the released version of the game apparently crashes if you leave it running for some amount of time over 10 hours (which wasn’t usually a problem in practice except for very long speedruns until it was released on Wii U Virtual Console and people started using save states which don’t reset the timer the same way as turning the game off and on does).
It may have done better if Nintendo didn't take so long to develop and release it.
I bought one on eBay, and it's a quirky device. It's essentially a glorified floppy drive with proprietary disks, and it makes loud, albeit amusing scanning noises.
Every Nintendo console prior to the Wii had some sort of "expansion" or upgrade capability for future peripherals to be added. Hardware revisions took the place of this model.
Man, it sounds like it was in development hell. I wish we could get somebody to talk about what it was like developing the 64DD and what problems they ran into. Probably won't ever happen, but I can dream.
Plus, adding more texture memory would have meant respinning the RCP silicon. That would have been a significant expense for marginal returns.
Besides, Nintendo had already sold a memory upgrade in the form of the Expansion Pak. A second memory upgrade which couldn't even be installed into existing consoles would have been a very difficult sell, and could have soured customers on their future consoles. ("Why buy a Game Cube when they'll just release a Game Cube Plus next year?")