I guess I expect more from the baseline rules. If I'm rewriting the mechanics because it suits my interests better, that's very different from the game-makers telling me "what's the problem, just keep seeing them as half-elves, it doesn't matter that they work exactly the same as [human/elf]." The former is my active choice to leave behind a distinction that matters in search of something better. The latter is the creators forcing me to give up something I usually like, because it's...inconvenient for them, I guess? It doesn't sit well with me.
Now, maybe if things had always been that way, so there would never be a feeling of loss, I wouldn't mind it. But the history of a thing matters, even if maybe I could have accepted the proposed change if it were a fresh choice without that history.
Frankly, if 5.5e doesn't include half-elves...I'll probably just ask DMs if they'll let me use the 5e half-elf rules with whatever tweaks are required to keep things on par (which, per the "backwards compatible" claim, shouldn't be difficult or result in excessive power.)
Edit: Further, the way 5e has implemented the whole "dragonmarked house" thing is subraces within a given race. They'll have to actually rewrite that if "half-elf" actually means "you use human rules" or "you use elf rules," because now that means dragonmarks have to be race-agnostic subraces--they need to work with both the baseline human traits and the baseline elven traits. Or there need to be different versions, one for each. Or they have to rework the entire Dragonmark rules to replace them with something else that won't intersect with race at all anymore.