You would consider halting agreements during investigations which would cause losing millions of income... and what if they are acquitted, which seems to be the case with most of these allegations? Who would reimburse them?
Thats a tough one cause there is no scenario in which no one gets harmed.
On the one hand, brands will wanna pull sponsorships not by law but out of fear that their brand will get associated with the celebrity's image. Thats reasonable from their perspective but as you point out correctly is unfair to the celebrity if they are indeed innocent.
Similarly people in real life will shun that person and again for good reason. If its someone at your work thats alleged to have raped/murdered/whatever, and you dont know if its true or not, would you hang out with that person and risk being a victim? No, you'd avoid him because if he is a criminal he is a danger to those around him. So naturally people will shun an accused person and they'd be placed on leave or fired etc.. But again if he is actually innocent (and we just dont know yet) then its really unfair to him.
So brands and even regular people are being reasonable in protecting themselves when they distance themselves from a celeb based on what looks like a semi-credible allegation. But if that dude is actually innocent, then thats incredibly unfair.
The next logical questions then are:
1) Whats the probability of being falsely accused in a manner that would convince enough people for brands/people to shun you? (as opposed to a accused in a tweet from an anonymous account with the Barca flag). I think the probability is very low. Even Aziz Ansari in trigger happy hollywood still has a career because not enough people were convinced he did a big wrong. From all the celebs that were accused of rape and/or assault in that so called witchhunt, how many turned out to be innocent so far? A tiny minority?
2) Assuming the courts dont have enough evidence to go by and public opinion makes the wrong judgment, who is in a worse state: (a) a rape victim whose rapist walks away untouched continuing to be super rich & famous and loved by millions etc... or (b) A wrongly accused celebrity losing his career over a crime they never did?
Both are bad but the Ronaldos, Weinsteins, Spacey's etc.. will continue to be millionaires because they have already made millions even if it all comes to a stop now. It still sucks to lose a brilliant career over a false accusation dont get me wrong its super unjust. The point I am making is that anyone in a position to go through a Metoo campaign is not your typical average Joe, its an already established millionaire who will still have more money and fame than both of us combined despite the accusation.
If the public gets it wrong and say disbelieve an accuser when a rape actually took place, that person's life wasnt too good already and the trauma is multiplied. Thats not to mention the swarm of filth that will attack them and send hate mail and bully them online and make memes about their rape online.
So yes being wrongfully accused and losing your career over it is incredibly unjust and unfair, its just a) super rare to actually happen even in #Metoo Era, b) to even qualify for a Metoo campaign you have to already be a rich famous celeb with sponsorships and everything.. you'll be financially fine forever, c) I'd rather have a big celebrity wrongfully lose their career in the rare occasion where public opinion destroys an innocent person's career than having rape victims watch their rapists make millions of dollars and be worshiped by fans every day.