35 bucks! Wow, I must have been ripped off. The yanks pay as much in US dollars, so you've done really well.
As for the gunk buildup, nobody is suggesting that it's not a very important factor. Even with heaps of wear in the bushings it's the gunk that creates the load that eventually strips the teeth. However, you will find that there are blokes who have run the best synthetic oil and meticulously maintained their new valves but still seen them fail again after no time at all.
Compare this to the '95-and-newer design that just don't fail like ours do, gunk and all. I still reckon it's at least partially a bushing wear problem, though I concede that gunk plays a big part and also the poor alignment of teeth (my new valve teeth didn't even align vertically with the rack -- I wonder if they use the same valve for 250's or something).
The centre valve is made of steel, so it doesn't suffer the same problems.
And by the way, the previous owner may or may not have installed the left valve incorrectly. I found that mine had rotated on its own as the teeth got stripped off, and was functioning out of sync too.