That seems like a false dichotomy. Someone who's a pollyanna even about horrendous abominations is a problem in one way, someone who's indiscriminately contemptuous of reasonably useful code is a problem in another way. Software projects (or teams, or companies) can bog down or die in more than one way. Driving off people who are doing reasonably good work is one way, tolerating unreasonably low standards is another.
And also, correct criticism is not particularly "bringing" negativity. If there is something unacceptably technically bad, and you want to point to the person or people who brought the negativity, you'd do better to point to the original creators and/or the chain of people who have signed off on it since then, rather than pointing to the person who criticizes the problem. The irreverent child did not bring the negativity to the Emperor's famous tailoring fiasco.
Someone who reasonably reliably distinguishes between good and bad is useful. Making the distinction correctly is generally much more important than the style of expression. It's easy to hire pollyannas (or, indeed, to err in the opposite direction by hiring hard-driving abrasive jerks) who don't have the tech skills to make the distinction correctly and just fall back to emoting in their preferred style. It works better to hire the smaller fraction of people who can do a good job, and who can recognize a good job. When you do that, it doesn't solve all style-related abrasiveness problems, but it does help mitigate them. By the time you've worked with someone for a while you learn to translate A's "this is unacceptable" as synonymous with B's "this is flaming rancid goat barf". (And axiomatic though it seems to be to you, in my experience saying "flaming rancid goat barf" is not useful evidence of unwillingness to learn and teach.) Both A and B are useful as long as they're reasonably reliably correct. Much bigger problems are C who says "this is a good effort" about everything, and D who flips between praise and condemnation based on issues other than technical quality (randomness, politics, mood swings, whatever).
I totally agree that not addressing issues and not bring (constructive!) criticism is a poor strategy. But your persons A and B have the same problem: they're critical without being providing any actual information. Person B just compounds it by also being a jerk.
As a programmer/businessperson, I'm not interested in making people feel good about poor quality work. I'm interested in people who want to improve themselves and the overall product, and ideally, enjoy the process. I just don't see how insulting code or people helps. I think it's perfectly possible to hold high standards on code and respectful conduct, and it's perfectly possible for people to rise to those standards, guided by constructive criticism.