I'm really glad you brought that up, because it's an interesting dynamic. Industries generally know how to deal with companies and senators. People like Gorhill though are hard to control using conventional strategies, because they're not getting any money out of it, they don't have supply chains or a duty to investors that you can exploit. They're just people doing stuff.
This can obviously be both a positive and a negative. But when people talk about competitive and regulatory forces, it's important to remember that some forces also come from outside of the system.
Adblocking is legal, but another good (less legal) example is Sci-hub. Sci-hub is just as large of an actor in the debate about academic publishing as any other official institution, but it refuses to be subject to the same rules as those institutions. If you're a traditional publisher, you can't reason with Sci-hub. None of the conventional strategies you would use on a competitor work for Sci-hub.
In general, if you're a business, you would prefer to only deal with other businesses and (to an extent) the government. Purely ideological actors, or people who don't pay attention to the conventional rules of engagement are much more annoying. You have to find some kind of leverage over those people so they can't cause as much trouble.
I don't doubt that part of the reason Google is locking down extensions is for security. I'm also sure that there are executives at Google who look at locked-down extension APIs and the inability to side-load extensions as a way of locking out people like Gorhill, and making sure the debate over acceptable ads is primarily restricted to industry players. For them, the fact that these policies improve some aspects of security is just a bonus.