That's like saying that older music is better than new music because people are still listening to them today, but we probably won't be listening to modern music several decades from now. And, of course, people say the same thing with films. Whether these statements are true or not, they're annoying.
To say that Call of Duty 4 and Halo 3 play even close to the same way shows how little you know. There's more similarities between older games due to their simplicity anyways. Heck, some older games were just complete ripoffs of other games branded with a popular name (see Popeye). We see these sorts of things today, but they generaly aren't multi-million sellers. Although the classic games you listed were different, what's the big differences between them? You can't just list a couple of retro games and say "Well, they're different because I said so" and then list 4 modern games and say "They're the same because I said so."
The retro gaming audience consists of two types of people anyways:
1. Nostalgic people who want to play classic games.
2. Older folks who thought that gaming got too complicated with the NES pad.
I'm sure there are other types of people who play these games, but that's the main audience. We could very well be playing Call of Duty 4, Wii Sports, and other 7th generation classics several generations from now. Even though it seems like a long time, many of the games we consider classics released only 20-30 years ago. I could easily see people playing our modern games 20-30 years from now.
You want 4 shooters today that have a lot of differences? Halo 3, Call of Duty 4, Gears of War, and Team Fortress 2. Even though the list you showed was appropriate for my point (that modern shooters are different from each other), I felt like showing that you that there are plenty of shooters today and they have a lot of variety to offer. Stop looking through rose-tinted glasses.