A service oriented architecture is not a bad thing. It has its benefits as well as it’s downsides. The key is to know when to use the pattern and when not to.
I’m not sure what exactly “modular monolith” means but I’m guessing it’s some kind of mono repo with different services that share a large core but can scale independently. At Shopify’s scale I doubt you can just use a single service and database and not run into problems (both technical and development productivity wise).
The secret is keeping performance in mind throughout your whole tech stack and in your application code. Do that, and you might not even need to scale beyond a couple machines for redundancy, depending on your SLOs.
I’ve said this a couple times before, but it bears repeating - nginx can easily serve a million 1 kB static files per second over TLS from just one machine with a modern Xeon/EPYC CPU. Serving dynamic content doesn’t have to perform any worse than one or two orders of magnitude below that.