In my sample of one household - its true that we've started buying old favourites. Some are comfort food, old pleasures to eat that remind you of simpler times, but also the amount of cooking from scratch and baking has grown significantly.

Will the resurgence last, and will challengers fade away? 

I don't think its that simple. 

Eating more at home during these times also brings with it a consciousness about health. True - gone are the high-protein, plant-based, keto, vegan snacks grabbed from a kiosk on the commute - but in are more vegetables; more cooking from scratch; more pleasure from family sitting around the same table sharing the same food. 

The resurgence of indulgent old favourites is likely to pass, the success of challengers will be determined by how they can embrace the new consumer that will emerge and provide food that people desire at a price (health and financial) they will pay. 

If they can get this sorted - space on the retailers shelves will not be a problem.