Absolutely nonsensical view, if that's the case then why did many outright disregard De Zerbi when Brighton continue to sell their best assets across a multitude of positions with low quality replacements for instance.
Bruno going is a loss to the team from an individual standpoint, but the manager needs to creatively implement a functional system at United irrespective that the team plays as a collective with less impetus on individuals having a significantly higher influence above their peers but a cohesive contribution. This is why City / Arsenal have distanced themselves from the competition.
The problem is United under the current coaching establishment are still struggling to formulate a consistent ideology that's proven successful, therefore the loss of a key player is inconceivable.
I don't know exactly what happened at Brighton but they do have a policy of constantly selling players off which makes it impossible for a manager to succeed past a certain level. But this is different. United aren't a selling club but they do lose key players and when it happens they rarely replace them with players whom, at least at the time, aren't equally good. That doesn't mean that we just write the season off.
Let's have a look when these things happened
Ince, Kanchelskis and Hughes. Hughes wasn't such a hit because we had Cole but for the rest it was a tragedy. Scholes was, at the time a forward and Beckham was nowhere near to Kanchelskis yet. Against Wimbledon, SAF got so frustrated with Becks that he considered substituting him for good. Then that goal happened and its all history
Cantona. We replaced him with Teddy, a clear downgrade
Beckham: We replaced him with an 18 year old Ronaldo. The media in Italy laughed their arse off at the deal. They said that United spent 12m on a lanky 18 year old who'll dribble his way to the corner flag. Ronaldo was nowhere near as good as Beckham at first but SAF coached him to become an upgrade.
Stam: we replaced him with an ancient Blanc. A clear downgrade.
Ronaldo: we replaced him with Valencia. A clear downgrade
SAF is not alone. Lippi lost Baggio and Zidane for example. Klopp lost Coutinho. Shit happens, players leave but top managers find solutions and bridge gaps with something called coaching. Surely his Eredivisie entourage can bridge the void left by Ole's signing right? Ok that was tongue to cheek but seriously, writing of a season because of Bruno is extreme.