It's true that Pietrangelo hitting the market doesn't necessarily mean he's leaving. An eight year contract is no longer applicable then though. That evens out the playing field in that aspect. I don't think retaining both Pietrangelo and Parayko is an option for cap reasons and maybe also it not being ideal to keep playing an RHD on his offhand side. Parayko does seem to be too important of a player to trade for futures, unless options are that limited or the futures are that good. STL would essentially have to retain one of the two defenders though. Too much of a blow to lose both.
Something to keep in mind here is that as per the CBA, teams are allowed to temporarily exceed the hard cap ceiling by
+10% in the offseason and then need only to be cap compliant by the time the next 2020-21 season starts. (rumorued Dec).
With a $81.5 mil flat cap that gives the Blues and any team
+$8.15 mil above their current cap space limits.. Blues could sign Pietrangelo to an 8 year X $8 mil deal today and nothing could prevent that as its completely within the rules and likely a desired outcome to boot.
As you pointed out only Blues can offer the 8th year and all others only 7 years which puts the Blues in the drivers seat on term and then therefore contract amount as a $8 X 8 ($64 mil) > $8 X 7 ($56 mil) UFA offer at same AAV. On the UFA market other teams would need to offer > $ 9 mil per (ie $9.14 mil X 7 = $64 mil) for AP to receive similar money to his own team to stay.
So moving Parayko is not mandatory nor pressing need to re-sign Pietrangelo today, and they know at any time thereafter once he is secured, they can always listen to offers for Colton and likely have lots of interest league wide. If AP were leaving and only Blues management would know this at present based on contract discussion internally at this time then dealing Parayko now or any time wouldn't make much sense at all from their standpoint and move on from both their twin towers on D. Besides a recent Stanley Cup team 2019 doesn't move on from both their top 2 dmen if they wish to continue to compete for more Stanley Cups in the present.