As someone who worked in an all pairing shop for two years (run by a bunch of former Pivots), I can confirm it’s pretty draining, though my experience wasn’t nearly as bad.

I actually liked pairing a lot of the time, and still think it’s an excellent way to onboard into a new code base, even in a non pairing shop (I even miss it sometimes!). For me, the main drawback after a while was that I felt like I was less efficient when pairing on stuff I knew how to do, and my personal growth as an engineer was stunted because I could always lean on a more experienced pair. Since that was my first eng gig, my second job had quite a steep learning curve as I had to figure out how to debug and navigate on my own, without talking through it out loud.