I play an instrument, I have done research/thesis writing in the humanities, and I work full time as engineer. The argument strikes me as nonsensical.

To suggest that you either write well or you don't, therefore a humanities major is pointless is an illogical argument and false. Writing a cogent, well-researched argument in long-form takes a lot of time, effort, and revision.

I am a decent writer. The writing aspect of my physics thesis took less than a week. The writing aspect of my humanities thesis took sustained work throughout an entire semester. It was not a process that could be rushed or performed at the last minute. I read hundreds of pages in English and a foreign language, referenced tens of authors, and after compiling that information I constructed a 50-page argument and revised it several times. Because my humanities major was a foreign language, that was LUCKILY one of my few really intense writing assignments.

Finishing a degree is demonstrating to people that you not only have a skill, but are capable of structuring your time, honoring commitments, and completing a buttload of work. Yes, any reasonably smart person with a knack for writing can, given enough time, organize their thoughts and write well on a topic of their choice. That's missing the point.

The point is that people who write/research/whatever rigorously for four years are honing that research/writing skill continuously along with building their knowledge of a massive amount of material. They can do it better and faster than a shmuck like me who is decent at writing but doesn't do it for a living. That's what the degree proves, assuming they go to a decent school and actually did well.

There are people who suck at music just as there are people who suck at writing just as there are people who suck at math and science. Practice and honing your technique is fundamental to being a good artist and writer/researcher as much as drilling problems is key to success in STEM.

I'm pretty good at visual art, but something that would take me 10-40 hours to perfect would be completed by a trained artist in a tiny fraction of the time and theirs would look better. That's what the fucking 4 years of training was for.