I still think the term progressive is kinda pretentious.

Perhaps, but I don't think that you should judge a band based on an appelation that was applied to them. Now, if some band refers to itself as a ``progressive rock'' band, then go ahead. Most bands that feel it necessary to pigeonhole themselves in such a manner usually suck anyway. Most real artists try to avoid defining themselves.

I think your explanation that these terms are very time-specific (Progressive = 70's, Alternative = 90's, etc.) is helping me understand the definition better. But that makes me wonder how newer artists could possibly be called progressive when several other of these evolutionary phases of music have already come to pass.

My point is that these bands were progressive or alternative at the time, by the usual definitions of those words. But there tended to be a commonality of the sounds of the bands, and that sound then became known as ``progressive'' or ``alternative'', and the original meanings of those words were lost. A definition might look something like:

alternative rock(n): the genre of music largely defined by rock bands of the mid 80s to early 90s who were seen as an alternative to the popular music of the time, but quickly became the status quo

alternative (adj): having to do with or sounding like alternative rock

But you're right, and it's an issue that friends of mine have harped on for years; what is it an alternative to? It was an alternative. Now it's just a name.

But to actually bother answering your question, it's because the newer ``progressive'' bands sound like the old progressive bands. Just forget that ``progressive'' has a meaning of its own and think of it as a name. It doesn't follow that a composer would not be considered baroque just because he was born in the 20th century, nor would it make sense to not see Stevie Ray Vaughn as a blues artist just because he came after some other offshoots of that genre.