We have to make some compromise with any player we buy. Since "more attractive" clubs are ahead of us in the food chain, we have to buy players who are very young and unproven, older and past their prime, washouts at other clubs, injury histories, etc., ande/or overpay significantly. Players with enough of a track record to be attractive to other clubs, but not prohibitively expensive, and in their prime are simply not coming in enough numbers to be counted upon. Richarlison being the closest to that profile over the past few seasons. There aren't likely to be more than one of those every few years.
I agree that Onana is very raw. The potential is there. He is by all accounts a hard-working and humble guy, which counts a lot. He may pan out. He may not. 30M for a player of his profile was good business IMO. So, as it turns out, were Myko and Patterson, on the evidence we have so far. Lots of room to develop and the flaws of Myko in particular have been pointed out here, but I'd rather have the two of them than Digne for this season and the next two or three. That was a good sale.
We've been screaming for years not to buy castoffs from Sky Six clubs. This is the alternative. It has to be the way.