It sucks that you didn't get those books until the quote-enqoute "last minute", but heres some things to consider:
1) Fluff is easily transferable between editions.
2) Nothing stops you from playing more 3/3.5 games.
3) Those books cited came out a good bit before two days before the announcement. DotU was 05/07, Codexes were 06/06 and 12/06. You have to acknowledge that even the most recent one: DotU, would have given over a year of playtime had it been bough when it was released.
4) Most of the more recent books like say Elder Evils are actually fairly edition proof. I'm actually planning to use a good bit of stuff from it for my 4E campaign when I get around to DMing. The stat blocks are obviously not straight out of the book usable, but the way they have the monsters set up is fairly similar thematically to some of the 4E concepts, and I don't imagine much trouble converting them.
If you feel that you've still got some play left in your 3/3.5 books, don't make 4e stop you from playing with them. Play 3/3.5 as long as you are still having fun. And if someone has all the 4E books and wants to DM a game, go in with an open mind and give it a whirl playing. If you like it, cool, you know have another game to get some fun out of it. If you don't, no harm no foul, you still have plenty of 3/3.5 stuff to last you for quite awhile.