Q: Why Apple Can't prevent my macOS Sierra download from being lost in this 21st century...

If your Wi-Fi router has problem, it has nothing to do with Apple.

Buy a more stable router or talk to your provider about your Internet connection.

I think Apple should improve the App Store, full stop. I'm fed up with it losing connections.

But anyway.

When you lost the connection and reconnected did you look in your Purchases page on the App Store,

if a download gets interrupted you can often 'Resume' the download.

Also the OS is downloaded to your Applications folder for Install macOS Sierra GM.app, have a look and see if it is there, if you were lucky it may have just completed, size should be 4.78GBs, if it is smaller than this

delete it and start again.

Recommended settings for Wi-Fi routers and access points - Apple Support