Yes, well new technologies are like that. Sometimes they're wonderful, sometimes they're almost wonderful, and sometimes they're, well, not.
My understanding is that the problem with aluminum wiring is that aluminum's thermal coefficient of expansion causes connections to work loose from thermal cycling. Long lengths of cable pose no problem, but connections both tend to get hotter than straight lengths of cable, and connectors are less able to deal with the such heating without failure.
If there had been an imperative to continue using aluminum wire, I suspect someone would have come up with a way to do so safely and economically. Aluminum wire's price advantage over copper, however, is sufficiently small that in most cases mitigation would have to be impossibly cheap to be worthwhile.
The aluminum wire analogy does differ from the PEX pipe situation in at least one key regard, though: whereas aluminum's primary advantage as a wiring material was that the material was cheap, PEX appears to be superior to copper in a number of ways. If a PEX pipe freezes without breaking, its final cost--even if it has to someday be ripped out and replaced with something else--will be no worse than would have been copper.