When you actually try to think about it from an expected value perspective, Malo Gusto EV is probably below 30-40M so it's completely logic to sell at that price.
If you estimate him as a good player even giving a 20% chance by himself at getting champions league qualification (which is hugely optimistic - obviously the team is not the sum of each individual player but this is a first order approximation) you barely reach that value at all when assuming a reasonable number of years of contracts left.
It's easy from a supporter perspective to argue to refuse high $ offers and buy above market when it's not your own money at stake. You have to be a little more objective here.
Regarding the fact that they did not plan for a replacement, it is probably stupid to assume that they didn't think at all about it. Life is not binary (not all black or white), it's a continuum. Maybe they didn't think about it enough, maybe they did, no one knows, let's see.