It takes a lot to make me get off of Safari. About the only time I do is when I must have Flash for streaming services and then I use Chrome's integrated flash support.
I guess if MS adds all of the proprietary features of Edge on Windows so that macOS users can run locked in Windows web apps that might be a highly attractive reason but it seems they're not going to be doing that.
I'm wondering if they're using integrated Keychain support? I guess if it's build on Chromium perhaps not. Frankly for me that's a non starter.