It's most likely a combination of both. I'm not a huge fan of the divisive "normies" vs "whatever the hell we are" stance, but Reddit became what it is because it was poorly designed from the beginning to handle how rapidly it needed to scale. It was never envisioned when the project started as an internet killing behemoth, but ultimately that's what it became. Without in-built tools to manage that growth, Reddit succeeded because the community willed it to be and in spite of its own codebase.

What's happened to it now is likely correlated to a number of factors:

• Significant user growth as the popularity of the site among habitual internet users grew over time
• Positioning within popular culture - namely the practice of appending Google search queries with 'reddit' to improve results, which is common among people who otherwise don't browse the site at all
• Unchecked bot traffic with limited mechanisms to control or curtail the propagation of duplicative, low effort / value, incorrect, harmful, or misleading information on a massive scale
• A philosophical pivot from being a community driven by community to a company driven by a desire for profits
• Algorithmic manipulation of how content is displayed to maximize advertiser return at the expense of organic community dynamic shifts a combination of 1- a rapidly grown userbase, 2- positioning within popular culture (vis a vis, appending Google search queries with reddit to improve the results is common even among people who otherwise don't use Reddit)
• The hurt feelings of a CEO with an easily bruised ego

I think it's probably both.

I think the biggest decline was just before the 2016 US election.

That seemed to be the point at which the site hit "mainstream" and with that came a huge influx of new users, and with the influx of new users came the increase of corporate interest to advertise to the new massive audience.

r/all switched to be almost nothing but arguing US politics.

It's both. The normies are the reason the content and vibe sucked, and the corporate manipulation is what filled it with normies. Reddit had hit Eternal September for me nearly 6 years ago. I only stayed because there was literally nothing better at the time.

All I'll say is The time r/funny became a default sub, things started to go downhill and I had to start blocking subs from my page