I could easily see either side of the statements made on the future of the MK2/3 in this thread happening...
An updated menu version that was "good enough" would keep us (meaning MK2/3 users) sedated as far as that goes, and a wonderful new MK4 would be a great distraction from old hardware at that point...
I think bruceg might have hit the nail on the head in his statement that the hardware of the mk2/3 is what might be holding back using the SD card for running full speed roms and having complete compatibility, and if the developers start back from square one, using the knowledge they've learned from previous versions, they might be able to work around any limitations they've had in pre-mk4 versions straight off it's production line...
I am not holding my breath though, and until enough time has gone by (after the release of the MK4) so that these forums can be flooded left and right about MK4 issues, and until every problem of the MK4 is solved and everything is 100% working with it, will I then peer into my wallet to see how much I might have to upgrade my DS hardware.
Upgrades are great, and new toys are fun, but the 'zip ram' that was never used once in older MK models does worry me a bit, and makes me wonder if I'm using a hundered-dollar-plus toy that is on it's way to the land of extinction as I type.
Evolution happens, and it is a good thing... I'm just not prepared to be the one left holding the bag a second time, if the "next best thing" is right around the corner and I end up holding the bag this time (I can't imagine how those MK1 users feel at this point).