The idea that a referee would prioritise their bias against a club over their careers seems highly ludicrous. This notion can only be arrived at through extreme solipsism of fans projecting themselves onto everything and everyone (which unfortunately is a common theme). Players are fans, referees are fans, journalists are fans, they are all just like me, except they have different names and different jobs. They might be apparently different, but their raison d'etre, just like mine, is to see one club do well, and have another get all the unfettered misery it deserves. These refs, they can't sleep thinking about how much they hate Liverpool, surely, and they'd give their arm up to see United rise to glory again. Except they might be from Leeds, but then it's just more of the same stuff but with different names and kit colours.
The idea that not everyone is invested in football fandom to the same degree or in the same way is completely alien to most fans, which is why they label players who pick clubs based on the best working conditions 'mercenary' and they are constantly baffled that Luke Shaw or Ole Solskjaer could play for United when they are fans of Chelsea and Liverpool respectively. It can't be fathomed that for people who are professionals in an industry, fandom isn't really a thing in quite the same way.
You may have supported a club as a child, and there might be some lingering bias for it in an adult, but that is more a sympathy than a full on affective obsession. Raised a Toffee through and through, but lo and behold, when you need to make a career decision, when it's your livelihood at stake, you do what Wayne Rooney and Steven Gerrard did, and become a legend somewhere else entirely. These actions are unthinkable to a fan, because they would never do it, because their loyalty is eternal, it is what makes them fans.
Referees aren't fans. Not in the same way as you or me. If they have any lingering biases of the sort that is assumed, they need to be aware of them and effectively purge them, because it can only ever be an extreme hindrance to their career. These people aren't in charge of matches so that they can watch them and be entertained, they are there to earn a living, and to do that they have to be good at it. Of course within a culture that designates them the role of a piñata, it can be a bit of challenge to understand that they aren't trying to make bad decisions on purpose. But of course they are not, and their jobs are highly performance dependant, and there's a ladder you need to climb.
If this banal stuff were understood (and it can be if you only make the slightest of efforts to place yourself in someone else's shoes), we wouldn't be talking about referees having it in for a particular club, and absurd conspiracies such as all but united would not blossom.