I think there’s an actual M/P $4.99 minimum, but you can keep it on perpetual sale for $0.99 if you wanted. And to keep liveries separate from the core aircraft(so XB users could delete it should they choose).
They COULD always include them in an update for free direct to the aircraft, but some may not want them for whatever reason.
I think most WOULD pay for a $5 liveries package provided it was the only one and all new liveries updated to it. This would also let PMDG control the quality of them and update them all together should things change.