The OP is right - they should never take a game offline altogether. It isn't like they can't afford some limited support - or hand some servers to microsoft so they can keep it running on XBL as long as they keep selling it.
Seeing as they are pushing this in the XBOX fan appreciation sale on now:
with just the slightest note at the end of the blurb:
"Some multi-player functionalities may no longer be supported."
which most people are likely to miss.
Bit of a scam for them to keep selling it if they don't want to support it anymore....