I am a bit surprised to see so many thinking he is going to play at this level for 4+ years, and it is a bit of an unfair expectation to put on him. The only attacking midfielder I can name that has continued to play at a great level beyond the age of 33 is Luka Modric. The rest have either retired or regressed heavily between 28-33. Examples are Cantona, Zidane, Iniesta, Kaka, David Silva, Sneijder, Özil, Deco, Coutinho, Bernardo Silva, Aimar, De Bruyne, Guti, van der Vaart, Eriksen, Nasri, Cazorla, Hamsik, Müller, Nedved and more. In fact, Müller is the player that is closest to Bruno in terms of body type, work rate, fitness levels and injury history. An outstanding professional like Bruno, and even had the benefit of rotating much more than our Portuguese has, which has allowed him to play for longer, but even he is not at all close to the player he used to be, so why do so many of you seem to think Bruno can do it? If it was likely, then I presume top clubs would have been circling around us like vultures now.
As for us not being able to replace him; well, yes, that's the downside with aging players, and it goes for every club and every football player. No one escapes it. Real Madrid sold Ronaldo to Juventus for £105m at 33, and he was the second best player in the world. They didn't demand £400m from Juventus because they need to replace him with 3 Ronaldo-quality players. It is unrealistic, and while Bruno is not as good as Ronaldo was, it is also unrealistic to expect a like for like replacement for Bruno, because there are no one as good as him at what he does. Real Madrid have been unable to replace Kroos and Modric, because these players are not around, so they have to adapt, and so do we. The advantage Real Madrid has, is that they have basically an unlimited amount of money to spend in the transfer market, and we have a very limited amount.
Sure, if we were competing for the Premier League or Champions League, I'd keep him because he could be the difference maker, but where do we see ourselves next season? 6th? 9th? 11th? It isn't much of a difference, and by rejecting a £100m+ bid for a player in his early 30s, we are effectively delaying the rebuild of our club. I agree, it should never have reached this point in the first place, but here we are, and it is the incompetence of our previous board's fault, nothing we can do about it now. If we are going to live in the fear of being unable to rebuild with the money we get in, then we may as well just give up.
And just out of curiousity; Alexander Isak for 7 years or Bruno for 2-3? If Isak wanted to come here (yes, Liverpool and Arsenal are more attractive now, but you never know, we are still the biggest club) then the sale of Bruno could effectively work as a trade for Isak, a world class striker at the age of 25, yet to hit his prime and is both a goal scorer and a playmaker in one. It would be like starting over with Bruno again, and maybe even better.