Sure, but that's only part of the equation. A lot of it really comes down to where you ride and how, and how much you weigh. I'm no lightweight - I ride a bike with 531 fork blades among the rest of the tubing on 38mm 650b Compass EL tires, and many times I'm happy to see the fork blades flex as much as they do. Micro versus macro. On smooth-ish pavement the distinction is minor at best, but that's not where it matters. Once things get rough you want that extra level of give.

Your idea for a XC MTB 650b x 42mm does sound like a blast, I'm just saying 42mm tires don't default you to a cushy cloud no matter where you are.