I have to think Armstrong expects one of the 3 RHD to play on their left side in the Top 4, for this trade to make a whole lot of sense.
Reading into team comments, it sounds like its going to be Faulk, although Pietro volunteered to do it during the first pre-season game they played together (the only game Faulk played with the Blues) to give Faulk a cushion to learn the system first. If Faulk is comfortable playing on the left AND if he plays at a better level than other (cheaper) options on the team, it makes sense. Armstrong must have determined that it would be a reasonable role.
I do think Faulk is a significant step up over Parayko on the PP. Parayko has a really hard shot, but it is too slow in the release and not accurate enough to really be the threat we all want it to be. I noted that Dunn is the lone defender on the 1st PP unit, and Pietro/Faulk are a tandem on PP2. It will be interesting to see the contrast in the styled the 2 units play. I like having two units that do different things, conceptually.
What I don't really believe is that Armstrong acquired Faulk just to play 3rd pairing and special teams. Maybe he did? If so, it will give Parayko and Pietro some relief in terms of wear and tear. Maybe that's all it was, an opportunity to get a guy they really liked for peanuts. Maybe the extension was just necessary from a business side to get Faulk to sign now. It provides some cost certainty. Could they be entertaining trading Parayko (not immediately, but in the next 2-3 years)? I really don't know what the longer term plan is. But I do think the team is stacked pretty well to make another run this year.