Not always. Even cyanogenmod has abandoned many devices that could still be viable phones today. CM seems to focus mainly on the most popular phones for the latest releases, and in some cases, the devs for a particular make/model of device have just gone MIA, and development stagnates.
Yes, it seems like most phones are abandoned by cyanogenmod at about the same time the manufacturer does. Certainly, this was the case Mytouch 4G/HTC Glacier. The last manufacturer release (less than a year after I bought the phone) was Gingerbread. The last Cyanogenmod: also Gingerbread.
They're good with Google's phones and the most popular Samsung phones but anything else is a gamble even if it is supported at the time you buy the phone.