Counter argument is that, you do not know whether your system will go big next week. If it indeed does, now you do not have the Google/Meta/Amazon engineer legion to immediately scale it up without losing users[1]. Also, a gambit of scalable system initially helps to solve the later death-march, when management wants to ship the MVP in global production and then come howling when users are bouncing because the system was not designed for scale.
[1] Unlike early days, users are very quick to dismiss or leave immediately if the thing is breaking down and of course they will go rant at all social media outlets about the bad experience further making life hell for "build things that don't scale".
[1] Unlike early days, users are very quick to dismiss or leave immediately if the thing is breaking down and of course they will go rant at all social media outlets about the bad experience further making life hell for "build things that don't scale".