Did they really say "total backwards compatibility"?
Because I think you may be falling prey to exactly the same thing that people fell prey to back in the D&D Next playtest: taking mildly-worded statements as being much stronger than intended, and taking the moderately stronger statements at their strongest possible interpretation, when weaker meanings could be intended (or, well, finagled, as was clearly the case with the D&D Next playtest.)
Remember that "modularity" was initially described as allowing, at the very least, a new edition that could be calibrated solely with official rules options to match 1st edition preferences and style, or 3e preferences and style, or 4e preferences and style, all smoothly, with the quickly walked-back implication that you could carry characters from one such table to another and not experience any issues or even have multiple different styles in play at the same table at the same time. As I said, they very quickly walked that one back because they realized that that was almost certainly not possible, but it's quite clear that they wanted people to understand "modularity" in a strong and expansive sense, and a lot of people, myself included, felt they fell rather short of that goal.
So: Did they actually say, in exact words, "total backwards compatibility"? Or did they say something else?