Just to be clear... there are four-strokes that are stone reliable, the Honda XR400 being one of them.
You coud ride this bike for years with minimal maintenance, barely changing the oil or cleaning the filter and it would keep on keeping on.
The problems started when they introduced the modern four-strokes that rev higher than the two-strokes. The piston, crank, cam, and valves are moving so fast that it's inevitable that there will be a problem at some point.
Sure if you just "chug" around on one of the modern bikes and do regular maintenance that the bike could last a while. The problem becomes, when will it let go? There is no doubt, you will at some point reach the time when everything comes to a screeching halt. What do you do then?
Two options. A very expensive rebuild. Or park the bike.
Both of these options are bad for the sport in general and the rider in particular.
Don't get me wrong, two strokes can and do have problems as well. They do require maintenance to run the best they can. There is one huge difference. There are two-strokes that are 10 years, 20 years and more that are still running with a minimum of rebuilds. I'm not so sure that you see many 5 to 10 year old modern four-strokes all that much.
I know where I would rather invest my money!