I believe most of the preconception against Samsung-branded Android OS is based on their experience with older (pre S6) devices. I wholeheartedly agree they were bad at it back then (esp. the bloatware that poorly replicated the functionality of various stock apps), but they've since gotten significantly better with their UX.

Yes I would forgive anyone who used pre-s6 devices to think Samsung is some lackluster copycat with slow TouchWiz UI and terrible useability

Why not just flash the ROM and install vanilla Android instead of settling for TouchWiz and other third party bloatware?

I don't consider any of Samsung first party apps to be bloatware especially since I use most of them on a daily basis.

While you could consider all the Samsung apps extra and unnecessary - the Android ecosystem is a loose group of OEM's with varying degree of control over their flavor of Android. The base features are always there on all android phones but Samsung personalized theirs. It's a good decision despite android purists scoffing at it.

Pixel line of phones is there to satisfy those who really want stock that badly they're willing to buy a flagship phone with budget phone features

Because there is no vanilla Android ROM that works perfectly out of the box on all devices and you need a device with a unlocked bootloader to even that get far.