I'm going to play the devil's advocate here and defend Apple's decisions.
"It does't need to be thinner"
It is thinner because it is more aesthetically pleasing. It doesn't need to be, but they could, and it looks better, so why not?
"It's still fat, just in the back; there is no point to the thin edge"
While the whole machine isn't 5mm thin, it appears 5mm thin from most standard viewing angles. Until you are looking to it completely from the side, it seems that thin.
"Everyone uses a SuperDrive."
People who use them think everyone does, but the truth is, most people don't anymore, and the people who do now, won't in a couple years making it a useless, outdated feature in the computer. It adds weight, power-consumption, cost, and noise, and most people don't use it. The few people who still use it, can then buy the ultra-thin plug-n-play SuperDrive from Apple or even just stop using the outdated technology altogether.
"It's absurd that they would raise the price."
Chip prices have seen a spike recently. In addition, Apple added an additional 500GB and 4GB of RAM adding cost. Finally, with inflation, Apple can't keep the Mac the same price forever, it's guaranteed to guy up some time.
"It's a desktop machine, size and weight don't matter."
There are many factors that come into play when designing a product: environmental impact, shipping, material cost etc. By reducing the size and weight, they are making it easier to transport, making it more sustainable, cheaper to ship, and cheaper to source materials for.
"Apple sacrificed power for size."
The new iMac got the standard spec bump. It didn't get any groundbreaking upgrades but it never does. Stop complaining, they did what they always do.
The major complaint I do have about it is they have made it less user-upgradable and harder to repair. This is purely intended to make more money and hurts the users. While I can understand fusing the display to the glass to reduce glare, making other hardware difficult to replace is unnecessary and a bad road for apple to go down.