They may stop making 720p tvs in the future but that will have no effect on the ones we already have. All these fixed pixel tvs upconvert or downconvert a 720p signal or a 1080i signal to their own native resolution. Many shows are now broadcast in 1080i and we get them fine.
Not a problem and don't worry about it. PS: Don't take too much advice from the brother-in-law in relation to HDTV. A little knowledge can be a dangerous thing....and none is even worse.