So if all of those inwall speakers are sh*t* wich inwalls speakers should we use then?
Meridian, JBL Synthesis (SCL-1 to SCL-4), Wisdom Audio, Triad to name a few. No one is keeping you from putting on-wall speakers into walls either. As you can see in the video I linked above, you can put a huge Genelec into a wall as well. Or build baffle walls for "normal" speakers.
MK’s are used in so many professional studio’s, why is that? Because they are just average?
Studios use everything, in the least case to validate how a mix transfers to other systems.
In studios those monitors are usually used at very close distances with a single listener in a fixed position. This is something completely different than most situations at home, especially for surround and home theatre scenarios. You can easily see in the measurements when a speaker is not really suitable for such tasks. That doesn't mean it won't work, it's just very flawed and if one likes such flawed solution is again subjective. I've had plenty of people tell me that a 50-bucks DIY speaker (just putting drivers in a box) is good enough.
B&W 800 series don’t measure well but they sound just fine(for most people).
Again, subjectively people are happy with a lot of bad, or let's call it not optimal, solutions. Objectively measured B&W are bad speakers.
I used to be one of these people ages ago, B&W was very high up on my list. Why? Because I didn't know any better. The majority of products out there are crap, so it's actually really hard to find the good stuff. It's not like B&W is crap and everything else is good, it's just so hard to find the good stuff. And yet B&W is sold everywhere... because it's probably the one brand that dealers can make the most profit on, so it's totally understandable they push this stuff... to make money.