Just as a follow up to this - I'm finally getting around this (yeah I know) as the PRIs were finally shut down. When our SIP trunks went live, I updated the ARS digits dialed screen so that all legit calls point to the SIP trunk. Now when I go to delete the ARS routes pointing to the PRI trunk group, it mentions that the route is used in a route list.
(As a side, I also discovered that we had a trunk group set up for loops-start/pots trunks) - Trunk group #1
In looking at the route list - it appears list #1 was set to use PRI first then use POTS if that falls through, however in ARS digits dialed, there's absolutely nothing pointing to that list - so it safe to delete that list as well?