From business end of things, I think it will turn out to be very good for Inter if you ask me. At worst, they will have to search for a new goalkeeper but they will really make a great profit out of Onana sale if you ask me. Yes, they will lose a very strong goalkeeper but guess maybe they have someone already in mind to replace Onana in the squad.
It is a profitable business from free agents to be able to sell 50/55 million if this scenario occurs, while Inter Milan can buy a goalkeeper at a lower price but of course you are right it will not be easy for Inter to find a goalkeeper of Onana's class anymore because already have extraordinary abilities.
I think Inter Milan is currently considering it, if for example Inter is ready to let go then the negotiations from Manchester United won't take long because Erik Ten Hag still wants goalkeeper Onana to be included in their project plans with Manchester United.
The great profit doesn't get them very far. As ajiz138 pointed out correctly, it will not be easy for Inter Milan to find a goalkeeper of the same class.
Goalkeepers in general are hard to be found when they really want one who belongs to the top 10 or even top 5 in the world. I would say that even a top 10 goalkeeper is not available right now and if one was available, it would cost more than 50 million. I think it started with Alisson Becker that transfer fees for goalkeepers also went up and are now closer to 100 million than they are to 50 million.
But if Inter Milans wants to attack again in all competitions they have no choice but to find a goalkeeper of the same level.