In the Ryan air and United case I'd say there is some huge discounts to get the type back in the air mainstream again. If you can get 3 Max for the price of 1 Neo why not, even if it is slightly dodgy.... she'll be right.
I really don't understand any commercial sense in operating Max and A321s. As having A320s would mean one crew set for all esp with no gap filler like the a321 on Boeings scope for a while yet. And not just crew, containerised handling between flights etc. The Maxs must be a dam good deal to warrant overlooking those issues for a 757 replacement.