First picture is a close up of the fan blade assembly. There's a (blurry) stainless steel spindle with yet another bushing and o-ring at the base.
These o-rings help keep the lubicant inside the center of the fan. The bushing help with side-load tolerance.
Second picture shows the inside of the fan motor with the driving coils.
No doubt ball bearing lasts longer, but it shouldn't be said that a sleeve bearing fan doesn't last long. There's a lot of concern about side loads, etc., loss of lubricant, etc. But since a good fan should have 0 tolerance between the spindle and brass bushing, aka "bearing", and given that the relationship between the coils and magent in the fan blade assembly creates a magnetic field that actually allows the spindle to "float" within the bushing even if there was poor tolerance, there is virtually no friction at the spindle. Furthermore, the o-rings should contain the lubricant as well as keep dust and dirt out that can actually increase wear.
Bottom line, the quality of the fan is the biggest determining factor. If the o-rings do not seal well and/or there is more tolerance between the spindle and bushing, the fan will certainly wear out faster than a better made fan with tighter tolerances and better seals.