I'm almost certain that the Roku 2 series will be retired at some point but I'm sure Roku will continue to update the software/firmware for at least a few more years. The Roku is like any other computer like device, the software can be upgraded but eventually there will be a point where the hardware won't be able to keep up with newer software.
I'm 99.9% certain that the Roku 2 XS has already been discontinued. That is why it is selling for $1.99 more than the XD on Walmart.com. Since the HD and XD are still products, I'm sure that Roku will continue to support the XS well into the future. The XS is a great deal at $80.
I think the series 2 Rokus will be available for a while. The Roku XS will probably be the first to be discontinued in favor of the Roku 3 - which has the same features ad a Roku XS with a more powerful processor. But what it lacks is a breakout cable for composite video, it's HDMI-out only. I like having that option available, since I can plug it in to just about any TV anywhere. I also use the analog sound output to my stereo, since it doesn't have digital inputs.
I'd say that if what you really want is a Roku XS buy it soon, otherwise you may to have to choose between a Roku 3 (with no composite out) and a Roku LT, HD, or XD (With composite video and analog audio, but no ethernet or USB port). And the price on the XS is pretty good right now: Buy a Roku 2 XS
Why would you want to discontinue something that works just fine? I mean why not be able to keep the versions they make? For example, you have the play station, play station 2, play station 3, and now the play station 4. Why not keep the other ones that are not new in stock so others who can not afford the new version to get the older ones?