I don't think that many of the trailers today will last anywhere near as long as an Avion or an Airstream. Airstream does make some with slide outs, even though they are rare. The issue with the Airstream is the weak frame. They brag about the semi monocoque structure which uses the shell as part of the structure. The problem is that the shell has so many holes, including the windows, door and slide out, if applicable, that the shell doesn't have much strenght left to it.
Avion, on the other hand, didn't use the shell as an integral part of the structure. You don't have to remove the shell to change out the floor on an Avion. If I were looking for a used trailer to rebuild and have for a lifetime, it would be an Avion. I don't think there is one made today that will last longer.
As far as loss of space, I have a 34' Avion triple axle and it has enough room, in fact it probably has too much room. The more room you have the more you tend to bring. If you are traveling, you just need to bring what you will actually use.