After having seen how they are put together, I think if I had a serious failure I would just unbolt the whole drivetrain and let out the bottom of the car and change it out if I had to. I think eventually there will be enough salvage cars available to buy the drivetrain used to switch them out. So most likely I will drive mine into the ground, then redo it and drive it into the ground again. I usually buy new cars ever 18-24 months, but I am going to try and not do that anymore.
My 325iT has been a keeper, had it for a long time now...it's an 03, and the smart may very well prove fun enough to be a keeper too. So it will most likely remain in the stable unless my son steals it from me.
The only other vehicle I have ever really "kept" is my very first car, a 1984 Toyota that I bought brand new at age 17. It now has a gajillion miles on it, but still runs great. Everything else has come and gone because I got bored with them.