When a majority of a countries people are not paying for the country or (worse) profiting off the government, then they have no incentive to keep government spending under control. This leads to the increase of tax money coming from fewer and fewer citizens until the whole thing collapses. Paying for something means that you pay more attention and be more critical of failures.
It doesn't actually work that way, if you think about it. In an election your vote pretty much doesn't matter. That means that if you're just going to vote for your own self interest you might as well not bother getting out of bed in the morning. Instead people mostly vote in ways that let them tell themselves that they're good people. Empirically, this is pretty well backed. For instance there isn't much correlation between income and how people vote on taxes, or between age and how they vote on social security.
Of course people have little incentive to find out which policies will bring actual good results as opposed to those which superficially seem nice. Luckily politicians who hope to be reelected can't do anything that turns out to be too stupid in practice... some fraction of the time. And so we sort of muddle our way through things.
I was just talking about this issue last night with a couple friends. One is a teacher, and she said that every morning for the last week or so, she's been getting a list from the teachers' union of the candidates she should vote for. The other friend works for a non-profit, and she says that it's repeated to them in no uncertain terms that if they want to keep their jobs, they better vote a certain way; after all, their funding depends on it.
So while your post makes a lot of sense on an individual basis, it seems that self-interested groups are almost certainly a factor in the way people vote. It would be pretty easy to miss this looking at things like income or age; I'd be more interested in seeing studies based on union affiliation or some other sentient grouping (as opposed to a non-sentient group like "people over 60").
There is quite a class of people who vote for their own self interest (e.g. California Government Union employees). 2000 should be the biggest counter example to your vote doesn't count(1).
1) The history of the 2000 election actually makes a serious point about polling and the failure of a news organization (Fox) to realized polls hadn't actually closed in the panhandle of Florida (Central Time). That area of Florida is heavily Republican and lower voter turn out because of the pre-announced winner (Gore). A few more Democrats voting or a non-preannouncement could have prevented one of the stupidest episodes in US voting history. Vote like you've heard nothing.
That is probably what he meant, but it isn't rooted in reality.
If that was the case, then you would expect to see most of the rich as republicans, but in actuality a lot are democrats. Taxes are not the most important issue for a lot of people in the upper class.
Lets forget the social issues which can be drivers of that, and focus strictly on the numbers.
A rational person wants to maximize the amount of money they have in their pocket at the end of the day. If government policy had no influence on the economy then obviously the best way to do this is through lower taxes.
But what if government policy does have an influence on the economy? I would rather pay 35% taxes on $100,000 than 30% taxes on $80,000.
This effect gets magnified at the top, especially when bonuses are in play.
Believe it or not, history has shown the economy to be better under democrats than republicans in the post-war period, and this will only become further exacerbated because this data doesn't include what has happened in the last 2 years.
When a majority of a countries people are not paying for the country or (worse) profiting off the government, then they have no incentive to keep government spending under control. This leads to the increase of tax money coming from fewer and fewer citizens until the whole thing collapses. Paying for something means that you pay more attention and be more critical of failures.