I wouldn't assume the worst - I think it's perfectly possible for the folks at Bio to successfully tackle some of the issues people have with the ending. But don't just 'wait and see'. If there's going to be
any chance of this dlc providing what we'd like to have (retconning the Catalyst aside), then Bioware needs to know exactly what that is.
Okay, so they aren't going to retcon the Catalyst, but they could, for instance, allow for a happy ending within the framework of the current ending pretty easily, and one that doesn't necessarily destroy EDI and the Geth. After all, it's already implied that Shepard
can survive the destroy ending, it's possible to have EDI walk off the Normandy in the destroy ending too, so with high enough EMS and peace between Geth and Quarians, it could totally be possible to repair the Geth as well, (or at least suggest that this could happen one day). Why not? A chance at a happy ending, hopefully including a little epilogue cinematic of Shep with LI or friends, plus some filling in of the various gaps, would go a really long way to making a lot of people much more satisfied with the ending - and this can surely be done without changing the
core ending, since pretty much
all of this stuff has been implied in what we currently have anyway. It's surely also possible to add more conversation options to the Catalyst to allow us to understand it or argue with it, even if our arguments don't go anywhere - a better illusion of choice, again without changing anything fundamental to the ending. Eh, I'm wittering now.
The point is, I think it's great to take a pro-active stance here, to let Bioware know what it is you'd like to see. The Extended Cut can't be set in stone yet, since they haven't been working on it for that long. So don't complain, don't pre-judge, and don't assume the worst - but fight for the best anyway, since you've got nothing to lose