It’s how things work in Windows at least, - the clickable area of the X spans the entire corner. Would love to see this happen in Mate, it may require some recoding of the window decoration function however.
Thank you all for this very fruitful and engaging conversations!
What conclusions can we take from here?
Is there anything of the ideas discussed above that we can somehow implement in the near future?
Happy Easter! 
In response to people wanting a bigger hitbox for the close button, you can install Emerald standalone from an older Ubuntu repo and use that to decorate non-CSD windows with.
Therein lies one problem; GNOME’s Context-Sensitive decoration doesn’t respect Emerald. Also, for some reason mailcap gets messed because of how the emerald theme mimetype is handled during install, couldn’t find a resolution for that, otherwise it works. That, or, y’know, at the risk of sounding like an boob, learn to press <Alt> + <F4>? 
Yes I know it was a joke, but it might be worth your noting that not every user is fully functional or able-bodied in the same way that you might be. I’m one of those users. Some control options are inherently easier than others.
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I created some green icons, take a look!
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