Plenty that can afford do don't, and plenty that can't afford to do. But plenty are paying for the front cabins. Many years ago, the front cabins (at least US carriers) had few paid passengers, very little in the way of discounted fares, and full of upgrades. Now, discounted premium cabin fares are readily available, with some planning and luck, and with revenue-based programs, it can make sense to buy a higher fare from time to time to avoid taking an "extra trip" to keep status. And in the days of FFP diminishing returns, if you want to fly in comfort, it is easier to just pay for the fare to where and when you want to travel than be a ho to keep your status.