I mean, I used to work at a digital agency that pulled in way, way more than $50k MRR. It did high quality work across a range of services (copywriting, web development, graphic design, social media engagements, online advertising). Mostly it worked by having long term retainers with large clients (I remember being amazed that we wouldn't even really look at jobs under 50k). It had a lot of younger staff but the leadership was experienced and serious.
It was a business that was really doing well, with a huge network of contacts in the region. I'm not saying it was representative of all agencies - probably it was an exception to the rule. But I definitely got the sense that there were ways of making the agency model scale well beyond 50k/month.
The article's implicit assumption is that there's only one grown-up/owner to manage client relationships. If your company has N of those, I guess you can reach $50Nk MRR.
I don't know what precisely the definition of "agency" is, but I've also cofounded more than one service firm that drastically exceeded $50k MRR, and I still think this is a really interesting, insightful post. The number is least important part of the piece.
It was a business that was really doing well, with a huge network of contacts in the region. I'm not saying it was representative of all agencies - probably it was an exception to the rule. But I definitely got the sense that there were ways of making the agency model scale well beyond 50k/month.