At my day job, one of the justifications for us adopting microservices is that we want to horizontally scale.
We have fewer than 8,000 users.
Hubris, eh?
They had 400 users, serving roughly 40 requests per second. Their database was small enough to fit into RAM on my cell phone.
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I'm sure there are 100% valid reasons for billion dollar corporations to use microservices. But most of the uses of it that I've experienced personally were not really warranted for any technical reason, and were usually some combination of non-tech/non-product problems bleeding into the codebase. (Whether that's inexperience, mismanagement, communication issues, lack of leadership, political strife, tacit permission to silo oneself off, developer boredom, or whatever.)
At one extreme, you have applications that need horizontal scaling from day one (Scientific computing). At the other, a monolith serving 10^6 users from a single app instance.
For context: the product at my day job does something likely at an equivalent level of complexity to Shopify. It's just another SaaS business that handles payments, invoicing, etc. If anything, Shopify is more complex.
Keep in mind Shopify does scale horizontally in terms of servers. They are just scaling a monolithic application.
Those 600k shop owners results in Shopify's platform handling over 80,000 requests per second according to stats that are publicly available[0].
It's one of the largest scale Rails apps in production.