I'm not saying that 775 is not a viable socket, because it is still very capable, as is 1366, 1156, and 1155. Buy what you currently want, then take advantage of a platform upgrade when you're ready/able. I see no point in complaining that you can't run that latest 8 core CPU in a 5 year old motherboard. Would you be able to run the latest ram (DDR3)? No. How about the latest 6gb/s hardrives? no. How about USB 3.0? No. So what is the point in trying to upgrade the processor alone?
The point is not having to swap motherboards all the time. After getting a P5K, I haven't needed a new motherboard for years. I was able to put a low end Core2Duo in, then I was able to upgrade to a Q9400 later, which represented a big leap forward. I could have kept upgrading motherboards, gotten an X58 or a P45 or whatever, but I haven't needed to and it's been very low maintenance.
I don't want to be able to drop a Sandy Bridge in my LGA775 board by any means. Don't think that I'm complaining about that. I just don't want to have to buy a new motherboard and go through all of that hassle (reinstalling the OS, learning a new BIOS, etc.) every time I want to upgrade the CPU. Every other time, sure. Like I said:
I see no reason to shell out for every incremental update.
I don't want to buy a 2600K or whatever, and then have to get a new board with all the SAME features (USB 3.0 -> USB 3.0, SATA III -> SATA III, etc.) just to support a process shrink.