I see no reason to lock out the option to purchase tiers from expired Mission Passes. The deadline should only lock out the ability to obtain tiers from the expired Mission Pass through the Weekly Tier Missions and other ARKS Missions that give you tier points.
If the Mission Pass has reached its deadline, then the player's only option to getting the goodies exclusive to that Mission Pass would be purchasing the tiers with Star Gems... instead of being SOL and forever locked away from those exclusive items.
I think this would benefit both parties. Players will be able to get the option to obtain Mission Pass exclusive stuff that they missed out on by shelling out Star Gems and the company will be getting more revenue from Star Gem purchases.