The broken part on drills is usually the chuck or the planetary gears for me, or the battery goes and it costs almost the same to buy a new skin+battery set. I go to great lengths to find light drills and usually go 12 volts for that reason. I would never touch an all metal drill but each to their own I guess, swollen wrists have been very convincing for me as to the merits of plastic. I love plastic on the handle, casing and battery, I cringe when I see plastic gears though.
To be honest with you unless we're talking about some cast iron machine, or the type of hand tools whose design has been the same forever, I don't really want my tools to last a lifetime. I had drills 30 years ago, they were crap. Technology moves on and I want the new features every now and then.