In other countries, they do work, just not here in a bunch of cases (the auto workers are a worst case).
I'm not trying to justify the adversarial relationship the UAW takes with the big three - but as the old saying goes, it takes two to tango, and for a long time, neither side was willing to change the status quo. Not all unions are the same, many have what could more be described as a partnership.
I don't have much in the way of sympathy with the Big Three however, they rather than really negotiate when times were good, they agreed to absurd contract provisions - a little pain now, can save a whole lot of pain later.
The reason GM as you pointed out had 4.6 pensioners for every worker, is General Motors used to be a much much larger company - in 1970, GM had 395,000 UAW workers, in 1998, 210,000 and in 2007 had around 75,000, today (February of 2015) it has around 50,300. So in, 1998, it was roughly 1/2 its 1970 size, 2007, it was around 1/5th - today its around 1/7th.
1/5th is awful close to the 1:4.6 ratio you pointed out.
As far as the 20 years and you're out provisions? I have a friend who worked for GM for 20 years, he has - bad hearing, bad knees, bad ankles, bad feet - plus 15 years of exposure to chemicals in the paint booth - then another 5 of working in a stamping plant. He earned every dime of his pension, and paid for it with ill health, and a diminished quality of life for his remaining 20-30 years on the planet, because essentially he's a man in his early 40's with a body of someone 15 years his senior.