I'm sorry but permadeath is the only solution when it comes to stretching out the lifetime this game has. Ever thought what would happen when you reach 200 dwellers? The limit cap has withhold me from breeding dwellers when I reached 100 dwellers already. I am at 133 now because of the successes of the radio room.
I dislike the idea of the BOS kidnapping your dwellers too, though. If you don;t want your fav's to die, don't send 'em out is my advice. It would be cool to use some as cannon fodder and others as heros.