The whole point why you should worry from the beginning is so that you don't have to re-work everything and put a huge risk to the business when you have to do it.
You never actually know where your bottlenecks are going to be until they arrive and by designing everything with a "super scalable" architecture you will be making development ten times as painful and expensive as it needs to be while throwing away nice things that come "for free" and "just work" at the mid-low end like transactions.
Amazon and Netflix don't want to have to use their hideously complex and inefficient service architectures - they're forced to because of their scale.
Most people who engineer their systems for hyper-scale from the get-go never see a whiff of anything that looks remotely like high traffic. Often they go out of business before they get anywhere near that.
And once you get to serious scale, you really shouldn't still be running the code from back when you didn't really know what your business/product was.
Certainly. However, there's a difference between keeping scalability in mind, and actually implementing/maintaining a complex database scaling system at the onset of a project.
Edit: I would also add that most projects I've seen tend to undergo one or more large refactors / redesigns before growing to a size where complex DB scaling is needed. This is, of course, speaking from the perspective of a small startup or pet project. If you're writing a new feature for an existing product where you can assume that it'll have lots of users, then you would obviously build scaling right from the start.
Instead you appear to be advocating for over engineering a solution for a problem you don’t yet have where the opportunity costs are probably features customers can use. That is also a huge risk to a business, but one that is immediate.
You never actually know where your bottlenecks are going to be until they arrive and by designing everything with a "super scalable" architecture you will be making development ten times as painful and expensive as it needs to be while throwing away nice things that come "for free" and "just work" at the mid-low end like transactions.
Amazon and Netflix don't want to have to use their hideously complex and inefficient service architectures - they're forced to because of their scale.
Most people who engineer their systems for hyper-scale from the get-go never see a whiff of anything that looks remotely like high traffic. Often they go out of business before they get anywhere near that.
And once you get to serious scale, you really shouldn't still be running the code from back when you didn't really know what your business/product was.