Is the Nexus-S gonna be the top dog hardware, no. But it will always be up to date with the current OS, and not really needing any custom roms, just a root. Sure in two months from now, there will be dual cores etc...but they will also have Moto blur, or TouchWiz, or Sense UI, and then have Verizon Bing search, or ATT crap ware. You will have to run a custom rom on those just to get it back to a Nexus experience, and roms can get buggy or flaky. I like the virgin Nexus, even if she is a little slower.
Every Christmas time there will be a new Nexus, running the latest "pure" aOS, sort of like every summer there is a new iPhone. I hope this keeps going