The reason sleeve and FDB fans don't last as long when mounted horizontally is the very small thrust bearing surface area at the ends in comparison to the very large surface area along the axle. FDB fans are essentially sleeve bearings running in an oil bath, and grooves in the bearing are intended to circulate the oil once at operating speed--which will not be the case when starting up. So if the majority of the wear occurs on dry starts, plain FDB should do better if rarely turned off.
If frequently turned off, Noctua's magnet-suspended fans last longer horizontally as the thrust bearing is kept from touching on startup by the magnet, and they are rated 150,000hrs. In fan tests the $20 Be Quiet has the most airflow for the noise generated (most efficient) and those are rated 300,000hrs continuous use. That's 10x longer than my favorite cheapie FDB fan, the $4 Arctic F12.
In fact the F12 is rated for no longer life than a sleeve bearing, which proves that the life of a FDB fan is primarily determined by how good its oil seals are.
Ball-bearing fans are rated 2x as long at 60,000hrs, hold up better at high temperatures >50ºC, and of course don't care about orientation but do get noisier as they wear so you may end up replacing them just as often. However they are famous for never silently seizing up like sleeve bearings do. They do change their noise characteristics immediately if dropped so you may end up with some bad samples.