No, I don't think captchas are a reason people are leaving platforms, at least not in any large number. If the content of a platform is good, the (extremely slight) roadbump for people is not going to deter them.

Keep in mind that for the average person (and even the average techy), they will not even see the captcha, because they have enough tracking them across the web that it will 'autoresolve' them as a human. I only see captcha's nowadays when I am working on web scraping automation or browsing from my VPS.

Yes, I think captchas are the wrong solution. They annoy all clients, humans included.

IP+subnet based throttling on the other hand is a perfectly working solution, optionally with a proof-of-work or login requirement for intensive POST requests only. Why is it a problem if only a few requests per day are made per home IP? Just how bad and inefficient is the web server?

Bots are not a second class client. They were here before AI, and they, like AI, work for humans. For the most part, they're not trolling the web autonomously.