We can't say for sure Faulk had nothing to do with Pietrangelo leaving. Pietrangelo could have felt less secure with the Blues and slightly insulted by the Faulk signing, making him more stubborn on his demand for an NMC. Armstrong could have been less willing to negotiate with Pietrangelo because we had, in his mind, 2 top-4 RHD in Faulk and Parayko. I agree with you in the sense that the Faulk signing didn't completely preclude re-signing Petro. However, it could have been a factor on both sides in why a deal was not reached.
Your scenario would make sense if the team had decided not to try to re-sign Pietro. But they did try, to the extent that Armstrong was willing to make exceptions to his stance about NMC and bonus money (not to the degree Pietro wanted, but it’s still an unprecedented offer from Armstrong).
What you’re describing is actually assigning MORE responsibility to Pietro than Faulk. If Faulk being here hurt Pietro’s feelings, but there was still a will and adequate assets to re-sign Pietro....and he left anyway, That’s not on Faulk. That’s on Pietro. If I storm off HF because I’m angry I saw you posting here, who is at fault for my departure?
I can sympathize with your point to the extent that Faulk’s presence could have theoretically changed Armstrong’s threshold in negotiations. But when Pietro leaves for LESS money, that argument is undermined.