...I think a few of you here are a bit confused, and maybe basing your impression of tumblr as a whole only on what is posted here (which is understandable. It's the loud, batshit insane fuckers that people are going to pay attention to--not the folks quietly minding their own business). SJW types typically HATE porn (or at least anything that isn't drawn cartoon lesbians). We're talking about a bunch of socially-awkward shut-ins. They hate real human sexuality, because it reminds them that they'll never experience it. They're also the ones that complain about the amount of porn in fandoms, try to police tags, and often harass NSFW artists. If anything, they're likely celebrating that the "nasties" are "being chased out". They've got no reason to leave.
It's the normal people that are willing to see the further harmful ramifications of the staff's decision that are concerned, and even so, that concern is on principle. In terms of what has actually occurred, really nothing has changed, or stopped people from posting what they want while mercilessly mocking the staff.
I'm gonna have to disagree with you here. While there is a sub-section of people who were clearly in favor of the porn ban, it's mostly been relegated to a subsection of sex-repulsed asexuals (an already niche group even within Tumblr niches) and even they seem to realize coming out in favor of the ban would be social suicide and instead give wishy-washy statements like "the ban is bad and all but I won't be leaving since it doesn't personally effect me" and the blogging/reblogging a bunch of passive-aggressive "Not everything has to be about SEX people" soon after.
The general consensus I've found with Tumblrinas/SJWs is that the ban is bad because "muh queer erasure": to them it's sexist and transphobic because the policy only singles out female nipples, it's sexist and homophobic because the majority of porn is made by women and is shipping-related and therefore gay, it's sexist and homophobic because Tunblr's AI isn't sophisticated enough to actually recognize the examples that are supposed to be permitted as such and thus flags posts about sex education and women's health, it's racist & anti-semitic because nude art references & classic paintings have been getting flagged and hidden but the legit "kill all Jews" white nationalist Nazi blogs have been left completely untouched, and I even saw one person claiming the porn ban was "synonymous with rape" because Tumblr was taking away users' ability to consent to see porn and making the choice for them.
The only real hostility I've seen towards porn in this regard has been towards the porn bots - which seems to be the one thing Tumblr can agree on for several reasons from hating the main porn industry to hating the viruses such bots are riddled with.
And I'd also have to disagree that the concerns raised about Tumblr's new policy are only being made on principle. Between the porn bots getting smarter and more aggressive in their methods with the massive wave of them hacking user accounts recently, and regular artists finding entire posts were blocked because
"somewhere in the 100,000 reblogs MIGHT have included adult content" has already been causing problems for a lot of frustration with the staff seemingly making no efforts to resolve the matter. Not only that, but while the artists might not post any adult content themselves, a lot of them follow accounts revolving around art references - which include nudes - and a lot of those have been negatively affected by this. Hell, I'm already seeing reports that figure drawing and medical references have taken a pretty hard hit where they're technically still visible, but all their content except for text post updates and the like has been gutted. Not to mention some people who went through the repeal process and still got false-flagged has pissed off the webcomic artists where random pages got false-flagged, got false-flagged again during the repeal process because Tumblr's AI is useless, and now readers playing catch-up are missing chunks of the comic.
While I don't think any of this will lead to a mass exodus all at once - especially since the main user-base for Tumblr tends to be reactionaries & reposters rather than actual content creators - you can already see frustration brewing among the remaining artists with the shoddy AI, the skyrocketing activity from porn bots, and Tumblr staff seemingly still patting themselves on the ass for getting back on the App Store while being woefully unprepared for all the other consequences this new policy had. Most artists already post their content on several different sites, and while some might not be ideal because they're not made for art (though frankly neither was Tumblr) they're likely proving to be less frustrating than Tumblr, and they aren't gonna wait 6 months for Tumblr to get its shit together. And with how
Tumblr has been hemorrhaging users as early as October, they really couldn't afford to lose users over this.