Doesn't this tell us that their penalty on slow pages just wasn't harsh enough?

If publishing slow, bloated pages would reliably come at the cost of not being featured in the first page of Google results, sites would soon change their ways.

Maybe, maybe not. Rewriting your existing site is much riskier for an organization than adding a new site which gets its traffic from a new source, while your existing site continues to chug along as normal (albeit with gradually declining search traffic from Google as the AMP box takes more and more of the share).

I don't agree with your uncertainty here.

If Google imposed a seriously strong penalty against bloated websites, people would promptly stop making new bloated websites, and would invest in lessening the bloat of their existing sites.

I can't think of a more certain spur to action than Google imposing a threat of going undiscovered. It's perfectly normal for web companies to flout the GDPR, say, as it's laughably unenforced, but Google rankings are no joke.

edit: a few tweaks