I'm very much in the same boat, also joined around 2011. I didn't leave because of the API changes, I left because the website was degrading substantially as a byproduct of its userbase.
Lemmy contains so much of what made reddit special in the early days. It was primarily tech-proficient people who cultivated a strong community, held each other accountable, and valued science and evidence.
As more users came to reddit, the initial community diluted. Certain subreddits were still special and worth checking out, but the greater whole was too massive for its own good. Plus, I suspect a huge number of new users were teenagers and children, and their comments and maturity reflected that.
I knew it was basically over once I saw comments on subreddits that regularly made the front page with extremely obvious bigotry and racism. Incescent bashing of women. Comments that reflected the vile nature of the shit comments you'd see on Instagram. This was becoming all too common and was not being moderated. The remaining comments felt like washed out circle jerking or a complete lack of critical thinking.
The IPO was the nail in the coffin. No good could possibly come from that for the users of the site. Haven't been there for over a year and have zero regrets.