PIC
I think most airlines have no problem accepting 135/91 PIC in lieu of 121 or military PIC time. They are primarily looking for command experience. I agree, being a 91/135 PIC especially S/P is definitely more challenging for a pilot.
There is also a lot of unecessary pressure in those positions at times, from the client or the employer. The layers of assistance you allude to aid in making 121 operations one of the safest around. Essentially, it takes the "grey areas" out of the equation, most of the time anyway. Most of us at the 121 carriers have come here after many years at the 135/91 playing field.
Yes, I do agree that 121 flying can actually be quite boring! But I like it that way after 2 decades.