Football is one sport that has a lot of uncertainties. Inter really wanted to get him but with the way it is now, I think Inter have angrily pulled out of talks to sign him permanently after learning Chelsea were in negotiations with Juventus. Despite Lukaku's pledge of loyalty and his success with Inter Milan, Lukaku has agreed personal terms with Juventus. Lukaku wants the move and it does look like the deal will materialize.
Lukaku has actually just lost quite a lot of his playing skills which used to be very good, maybe Lukaku's decreased playing skills due to his aging age because now he is 30 years old.
Inter Milan shouldn't have to regret too much if they fail to have Lukaku because there are still many young players who are more powerful and better than Lukaku.
If Chelsea really let go of Lukaku for Juventus, what Inter Milan should do is look for other suitable player options and can be a good striker at their team.