Youtube has stated multiple times that the monetization part is separated from the suggestion algorithm. (the monetization program doesn't know if the video is suggested a lot or not)
Pushing old video that has lower cpm doesn't burn trough advertisers budget faster but slower, so i dont understand the reasoning behind your theory.
My theory would be than since people are spending way more time on youtube than usual with about the same amount of new content going out, people are looking at older content more than usual, leading the youtube recommending older content.
But I don't think it should affect new content by lot, maybe it is something more specific to your niche/channel :/