Hmm, that thought never occurred to me.

I had assumed that having specific interests would help me because I knew exactly what the school had to offer (courses, concentrations) instead of a vague "I want to go to your school because it has good CS" / "I want to go to your school because it is prestigious," where other high schoolers may have difficulty thinking of what to say. However, the admissions officers probably never heard of PL, which hurt me.

But, how is wanting to go to a school because of its strength in PL theory any different from a kid who say, likes AI and applied to a school because it has a strong AI program, or a bio major applicant who's really interested in molecular biology or something? I wouldn't think that people would see these kids negatively. (I'm not trying to argue with you, you bring up a possibility that I hadn't realized and I want to consider it.)

One problem is the competitiveness of the dream. If one is good at computers & math, and wants to go to college to do computers and math so they can contribute more computers and math to society, well the admissions committee mentally sticks them in the oversized pile of computer/math nerds who are all also overachievers at computers & math. And never mind that industry desperately wants these skills, academia is a warped place with its own agendas.

AI/Machine Learning are likely similarly competitive goals currently. No sure about biology, but the connection to medicine probably means it is a similarly problematic choice.

Another big issue could be the reasons behind your plans. Was it an extroverted story of benefiting society with your new skills, or an introverted story of learning about stuff that excites you for its own sake?

For my supplemental essays, I wrote about the importance of PL theory for software engineering, where you want to design programming languages that increase programmer productivity and ensure correctness, therefore leading to less bugs (e.g. security holes) and a benefit to society. I basically tried to appeal to the applied motivation behind PL theory. I don't know if you would consider this to be a benevolent motivation, I mean, it's not reducing inequality or curing cancer or anything like that.

Another problem with overly specific interests is you back yourself into a very small arena. If the math department is accepting 1,000 students but they only have one or two professors interested in PL in the department, you are no longer competing for 1 of 1,000 slots, you are instead competing for 1 of 10 or 30 slots.

It also struck me that you said you wanted to do undergraduate research. Did you mean you wanted to work as a research assistant- or did you say you wanted to conduct your own research...? I don't think the latter really happens. I could be out of date.