Let's imagine Facebook, we can partition the monolith by user, but you would need the entire code base (50+ million lines?) running in each process just in case a user wants to access that functionality. I'm not saying one can't build a billion dollar business using a monolith, but at some point the limit of what a single process can host might become a problem.

Things like Facebook and Google are at a level of scale where they need to do things entirely differently form everyone else though. e.g. for most companies, you'll get better database performance with monotonic keys so that writes are usually hitting the same pages. Once you reach a certain size (which very few companies/products do), you want the opposite so that none of your nodes get too hot/become a bottleneck. Unless you're at one of the largest companies, many of the things they do are the opposite of what you should be doing.

I agree that most will be fine with a monolith, I never said anything to the contrary. But let's not pretend that the limits don't exist and don't matter. They matter to my company (and we're not Facebook or Google, but we're far older and probably have more code).