They are one of the only things tying the Raid Lairs to Prestige Mode, and taking them away would make going Prestige much less worth it. Plus, Raids aren't seasonal activities, they're permanent. The ornaments aren't going to be replaced by different ornaments down the line, and the activities themselves aren't going to change beyond what's already happened. For these reasons, I think the ornaments should continue to be available for all players wishing to do the challenges.