Now that the phone has been rooted you can relax about losing support, guys like Cyanogen will have our back for a while.
But officially we are at least guaranteed Android 2.3, and most likely the one after that (maybe...). Android 3.0 is currently being speculated as Tablet oriented, so who knows the version that will be released for phones after 2.3.
Unfortunately what you are afraid of WILL happen. Since the myTouch and G2 released pretty close, I imagine they will lose support around the same time, and it's that way for ALL phones. Eventually the carrier will stop caring and move on to their next hardware generation, and that will be long before you stop caring. Thankfully the Android community is blessed with wonderful devs who help keep our devices up to date long after the carrier shows them the door.
If we didn't, you would lose support way before your next upgrade.