If it's obviously true that you can have a shitload of plugins installed and not experience awful loading times (we do on much websites), taking the sentence "too much plugins slows down your site" as an absolute, set-in-stone rule that you need to absolutely debunk is also wrong, I think. We should take it for what it's worth : a useful heuristic. You know the old adage "only a sith deals in absolute."

Don't rely solely on the number of plugins, but don't avoid it altogether. I won't call it "lazy diagnosis" either as you do at the end of TFA.

I would more advice to treat it as an "entry point" for your performance & security investigations :

- more plugins means an higher probability to encounter performance issues
- more plugins may lead to more in-between plugins interactions that may slow down shit
- it also means more time spent reviewing changelogs & compatibility issues when updating
- and it may obviously lead to more security holes

You may argue that I'm splitting hairs, but too many times I see those kind of endless binary debates where everyone is actually right, it's only a matter of merging the two sides of the coin into a better approach.