This is sway, i3 on wayland, a tilling window manager. Not some commercial piece of software that has to target the lowest common denominator to survive in the market. It's meant so that you configure it and it's very configurable. So it doesn't any defaults bar what's needed to launch the manager. The rest is up to you.
Configurability is great, but that isn't an excuse for having poor defaults. A child comment to yours mentions that Sway now uses a heuristic to find a "reasonable" default scaling factor, so it appears that Sway currently does the right thing.
The "standard" for anything today should be the physical display size divided by the pixel count to give the exact resolution for X and Y in DPI. Scaling should be relative to that physically correct resolution.
We've inherited a lot of baggage and odd conventions, some of which were wrong to begin with. I don't think we should be carrying on with it if we can do better. Having these scaling factors directly correspond to physical reality would be a start.
I'd rather just have the configuration file be simple and well documented and let me make the decision. My monitor running at maximum resolution is about 163 DPI, so an automated system could guess both ways. 200% scaling works well for me, and it's a single line of configuration, done once, and I don't have to worry about heuristics changing behind my back.
Why do you think it needs any of that? For that matter why do you think windows, mac, android, chromeOS etc. all need "explicit configuration"? They all simply set a reasonable default scale and then give you an easy way to pick a different one if you want.
Why? What? Why? The system exactly knows the DPI of all attached displays, why does it still need explicit configuration in 2020 to support HiDPI?!