I think the reaction was driven by four things:
1) The legal profession tries to instill a sense of ethics into the lawyers they train, and what DoNotPay was proposing violated that ethics. I don't want to overyhype the legal profession, but many (maybe even most) lawyers really do want to do the right thing, which by their lights means clients get the best representation possible. Which an LLM currently is not, so you get a visceral reaction from people at what they see as discussion of/advocacy of unethical conduct.
2) As a practical matter, it was likely to yield very bad outcomes for all concerned, including professional and legal consequences. Bluntly, DoNotPay was proposing to do something illegal, it really could have resulted in jail time, and possible disbarrment for any lawyers who were involved. For good or ill, judges have immense power to control what goes on in their courtrooms, and the risk of a judge taking offense to this is high. And the higher profile the case (and DoNotPay was offering $1m for a lawyer who would repeat whatever the chatbot said in a Supreme Court case) the higher the stakes. That really could be a career ending mistake for a junior lawyer (plus, almost by definition, anything that reaches the Supreme Court is important; bad representation could throw the result, with potentially terrible consequences for the country).
3) This is not, by any means, the first attempt to try and automate or streamline the provision of basic legal services. And it's a field ripe for such things; simple stuff like many rental agreements, employment contracts, sueing in small claims court, divorce agreements, etc., etc. all seem like they should be able to be generated by filling out a form and pressing a button, not hiring a high priced professional to craft a bespoke document. But over and over, such attempts have failed badly, and lawyers are getting reflexively defensive to anything that looks like that, not because they threaten their jobs, but because they keep being so terrible.
4) And DoNotPay looks a lot like these previous failures. See, eg, https://www.techdirt.com/2023/01/24/the-worlds-first-robot-l... which makes them look less like some cutting edge AI lawyer that will put an army of paralegals out of work, and more like yet another shitty site trying to resell Mechanical Turk at an enormous markup. Now, maybe what they're showing to the public is entirely different than what they want to use in courtrooms, but...it doesn't build confidence.
In many ways I think this is analagous to self driving cars. There is scope for enormous gains here, it's probably inevitable eventually, and it will probably put a lot of delivery drivers, taxi drivers, and truck drivers out of a job when it finally arrives. But right now deciding to hook your new self driving AI up to an unmarked truck and do a secret test in an unnamed US city without warning anyone or getting any licenses or permits is unethical and illegal, and you'd expect a very negative reaction if you announce your plans on Twitter. Especially if your website offers some software that they claim can control an RC car, but when you try it, it keeps steering into walls.
> I assume the real reason it was shot down was out of fear that it would work well
The next lawyer (or AI expert!) I see who thinks it would work well will be the first. There's just an enormous mismatch between the requirements, what chatbots in general seem to be capable of right now, and what DoNotPay in specific seems to be capable of. (Again, see the the Tech Dirt link.)
And again, note that while there's a pretty strong argument that it should be faster, easier, and cheaper to create a rental agreement (or whatever), DoNotPay was making a big deal out of how they wanted to argue in front of the Supreme Court. DoNotPay doesn't seem like they're going to nail the "draft a rental agreement for cheap" any time soon, but maybe they (or someone else) can. But handing a Supreme Court case? Obviously not. Now you might argue (and one certainly hopes!) the whole Supreme Court thing was just an unserious PR stunt, but when a company is making undifferentiated claims that their tech can do something semi-plausible and something entirely impossible, it makes the entire thing look like a scam created by or aimed at people who just don't know any better. Not a good look!
In short: I think the negative reaction is very, very understandable.