When a friction plate is new, the rivets are well below the surface of the friction material.
When it wears to the point that the rivets touch & start to wear, friction is reduced and the clutch starts to slip, accelerating wear until the slippage is bad enough that it's pulled for replacement.
The rivets do most of the grooving of the flywheel, and if not replaced/resurfaced when possible a new clutch will wear fast as it conforms to the grooved surface. That reduces new clutch life significantly.
Why do they wear? Like brakes there's some wear each time there's slippage during contact. No wear when engaged, but you "burn" a bit of clutch each time it's slipped to get moving. Minimizing slip when starting from a stop increases life, shifting while rolling requires NO slip when speeds are matched.
Trucks use low enough gearing that slip is minimal as the clutch is "dumped" at idle to get rolling, high performance clutch type in them that's NOT meant to be slipped at all in normal use.
Car clutches can last a long time in normal use, they can also be worn out fast when a lot of slip is used.
P.S. - "Riding" the clutch (resting a foot on the pedal) is a common cause for early wear. Not good for the release bearing, and terrible for the clutch as slip can be caused that's not even noticed.
Engine racing up in rpm but car not going any faster. The sensation engine is not hooked to rear wheels as much. A tachometer shows it instantly.
Your example of worn disc could be normal wear if it lasted a long time. They all eventually do that. Once the rivets hit then the clutch begins to slip since rivets cannot connect to flywheel as well as the clutch friction material does.
The only time I've noticed where my car was decelerating while the gas was pressed is on hills. I normally just decrease the gear to one lower to accelerate until the higher gear can be re-engaged again. But that is normal, correct?
When my clutch was replaced, I first had ford dealership remove it and get a quote for everything that went bad. They said I had to replace my clutch but my flywheel was in good shape. I then had the car towed to a family friend mechanic and he also said the flywheel was in good shape. So I think I can trust them both. Especially if the dealership said it was fine. You'd think they try to replace anything they could!!!!