It's not. It's kind of bonkers to pursue that when you have a lot of traffic, but it's a perfectly sane starting point until you know where the pain points are.
In general, the vast number of small shops chugging away with a tractably sized monolith aren't really participating in the conversation, just idly wondering what approach they'd take if they suddenly needed to scale up.
I'm not even sure it's bonkers if you have a lot of traffic. It depends on the nature of the traffic and how you define "a lot". In general, though, it's amazing how low latency a function call that can handle passing data back and forth within a memory page or a few cache lines is compared to inter-process communication, let alone network I/O.
The corollary to that is, it's amazing how far you can push vertical scaling if you're mindful of how you use memory. I've seen people scale single-process, single-threaded systems multiple orders of magnitude past the point where many people would say scale-out is an absolute necessity, just by being mindful of things like locality of reference and avoiding unnecessary copying.
In general, the vast number of small shops chugging away with a tractably sized monolith aren't really participating in the conversation, just idly wondering what approach they'd take if they suddenly needed to scale up.