If they hadn't given people the option, I don't think the site would be up today. I already saw one artist who wiped their account and left the site within a couple of hours of the announcement.
And they couldn't have picked a worse time to do this after the drama with the CEO banning a popular trans woman permanently off the site last week and threatening to sic the FBI on her for a "threat" of cartoon violence she made after the year-long harassment campaign she suffered had been ignored leading up to her being banned over a transition timeline picture of her face. The CEO then went on to spend hours going into trans women's dms to insist how he's not a transphobe after writing a post about how she had been banned for the "threat," not the picture, even though it had been known for a while by that point that she had been banned for that photo being "nsfw/sexual content," while he eventually started calling her "it." He then topped it off by chasing her and harassing her on Twitter, still insisting that her wishing cartoon violence was a tangible threat and posting the names of accounts she had had at various times on Tumblr. All while he was supposed to be on a 3-month vacation.
Between that and the rumors of this AI deal happening that popped up last week as well, people were already looking for alternative platforms. Allowing people to opt out is the least that they can do if they don't want to run off the users who keep the site running. I don't think many of them are happy that they even have to do that.