I often equate comfort to performance so the contact points on a bike might go into #1, but you could def just make a category regarding sizing or rider geo when talking saddles, stem and bar length, seat post, etc.
i think change in riding style opens a possibility of another category. I like the wheel conversion example and other examples might include stem/bar combo for DH capability, fork travel, droppers, etc.
so far for me it has been 1,2 and 3 haha. I replaced my cassette because it was worn out, I replaced my wheel set because I taco’d them as well as my pedals for the same reason. I bought new lock on grips because they were on sale and the end rings match my bike.
And like Jeff said, I have also replaced items for better fit// riding style. Fork upgrade, shorter stem and wider bars. (that set up has to be my favorite upgrade so far by the way.)
Curiosity would be another category. For example, my bar-stem combo are perfect for what I’m doing, but I’d love to demo a 35mm set up just for the sake of trying it. Clipless to flats out of curiosity, a 1x drivetrain out of curiosity, etc. Sure, you might be curious if it’ll perform better or look better, but sometimes there’s no reason to upgrade save the case of curiosity.
I have rebuilt my entire bike from factory nearly 2 times. Most due to reason 1, some part of 2 and a few items being reason 3. But I think curiosity is a valid category. One reason I went to 1x. I was curious mainly how it would effect my riding styel but, also wanted to shave weight a little, quite the ride and have less moving parts. Same reason I went to a short cage rear, I could have rode a medium or long… but was mainly curious as to how it would ride and shift. The added clearance was a benefit.
I have the same curiosity in regards the oval chaining. Not sure it will be a performance enhancer as much as a conversation piece. But I could see me making the purchase to see what it is about.
I mostly only replace parts when they break. I tend to adjust to new kinds of riding styles by just adjusting how I ride. when I grew up riding (on BMX bikes), you just saw a place to ride and went there. I never thought “oh, I need my xxxx width tires and my 1×9 gear set up and extra wide handle bars”. I just rode over whatever was in my way and adjusted with my body.
Granted, I don’t ride extreme DH or do any kind of racing, but even if i did, I think I would try it with what I had first to test my technique and skills.
…and I think that rung-what-ya-brung mentality still applies to the vast majority of riders and is the reason an all around trail bike is probably the best selling category on the market. I do feel certain components (wider tires, wider bars, fork travel and stanchion size) do lend themselves to better performance on certain terrain and with different riding styles but that would probably only significantly matter under race conditions or extremes of terrain such as XC vs DH.