There are approximately 1.4 million people serving in today's military along with 1.9 million family members. Of those 1.4 million, fewer than 15% will stick around at least 20 years. The highest rate is the Air Force at almost 30% and the lowest is in the Marine Corps, close to single digits.
America's Military Population - Population Reference Bureau
So your taxes are supporting maybe 200,000 pensions. Not bad considering that Americans haven't been drafted since 1973 and American soil hasn't been invaded since about 1942. It's hard to put a cost-effectiveness assessment on a guided-missile cruiser that costs billions to build & operate for 30 years and never fires a shot in anger. And I don't think we want to try to assess the cost-effectiveness of a ballistic missile submarine.
Here's some more numbers. In 2001, my last full year of military duty, my taxable pay was $59,839 and my untaxed allowances were $27,788. That year I was not eligible for a nuclear bonus, which back then was a taxable $25K/year. When I retired under the military's "Final Pay" system, my monthly check was $2568/month or $30,816/year. So in other words my "50% pension" was about 35% of my previous compensation, and 27% of what the average nuke rated. Note that the vast majority of today's military retire under the "High Three" system, which reduces the check by an additional 5%. And that's not counting deductions for the Survivor's Benefit Plan annuity.
Here's another interesting observation that needs further study. If the military's healthcare system is so good, then why are military retirees dying before the rest of the members of their demographic group? It's possible that the long hours and high-stress lifestyle are making us a taxpayer's bargain because we're not sucking up those pensions and healthcare benefits for as long as our contemporaries. And I'm still not sure that medical science really knows what 311 millirem of documented exposure to ionizing radiation will do to my longevity.
Those of you with "military benefits envy" could join occupations with the same disability/death rate. Then you, too, may be able to reap the benefits from being one of the survivors. Assuming, of course, that you're not tapped to volunteer to fill the ranks of the disabled or dead. And assuming that you live as long as you would have been expected to live if you hadn't embarked on such a hazardous lifestyle.
And if you want a COLA pension then go buy one from Vanguard. It might cost a bit more than the federal version because Vanguard doesn't injure or kill every umpteen thousandth applicant. But it's not outrageously expensive and it's these days it's actually recommended by former anti-annuity economists like Milevsky.
Nobody should join the military for the healthcare or the pension benefits. And those who don't join the military shouldn't whinge about the benefits.