Here's my burbling-into-the-void take:

Gargron's attitudes make sense in light of the techbro axiom "Bigger is Better; Growth is Good."

Every time his ideas come under fire, his first protest is "but this will help Mastodon grow!"

grow, grow, grow. bigger, bigger, bigger.

It's the same thing that underpins most social media platforms' toxicity issues. More users, more "engagement", is always privileged over other concerns.

Abuse prevention is valuable only insofar as it encourages More People to show up. If, on balance, creating a feature that would cause more abuse and harassment would overall help the platform win at Number Go Up, the abuse is considered an acceptable side effect.

Given that abusers and harassers are great for numbers--dogpiles consist of many active users generating lots of toots--favoring them over their victims gets to be very tempting.

Thus Gargron saying "it's their loss" when an instance blocks another instance for abuse. They're losing the numbers game! They're choosing not to be Bigger when they could be!

Thus Gargron saying "some people want tiny invite-only instances, fuck that". Why would you ever choose to be Small when you can be Big?

Thus Gargron's resistance to implementing local-instance-only toots. Federating makes numbers go up too! Notice how prominently the default instance frontpages display how many instances they're federating with, how many users, how many toots.

etc. etc. etc.