I think the gearbox bearing in question receives thrust from helical cut gears (ironically the helical cut as opposed to straight cut gears purpose is to reduce gear noise ) which due to their design produces a force along the axis of the shaft as the angle on gear teeth try to push gears apart along their axis. IMHO bearing is more likely to fail if car spends more time in lower gears, as this is where maximum thrust happens during acceleration. My wifes GD Jazz was a 'local car' and got the noise about 80,000 miles, my identical year GD was a 'commuter car' spending most of its time on 'A' and 60mph 'B' roads in 4th and 5th gears and mine still had no noise when I traded it in at around 120,000 miles.
Straight cut gears are stronger (but it is not the gears that fail, just the bearing) but are noisy, listen to transmission noise on BTCC cars or similar high performance cars (or in reverse on most cars) and that is the whining noise of straight cut gear teeth.
The main problem (and same with rear wheel bearing noise) is that car makers are using deep groove single row bearings instead of taper roller bearings, whether this is cheapness or just desire for lower friction I know not, but it does mean bearings under more stress and fail sooner than they should.