Picking up my phone to unlock my mac doesn't make anything easier. Adding a sensor to the keyboard will.
To this (and everyone else) complaining about how this won't make things easier:
(a) You can ALREADY get this today by installing MacID, a combo OSX+iOS+WatchOS app that provides this functionality. It costs $3.99 and is, IMHO, totally worth it. Buy on the iPhone store and download the Mac part separately [showing, yet again, what a cluster fsck is Apple's Mac store strategy --- lack of links to the iPhone store, and unwillingness to support certain types of apps --- but that's a rant for a different day.]
(b) If you use the watch to unlock, you just have to touch the watch face to unlock. It works REALLY nicely. The first two or three times it feels a little clumsy and slow until you figure out a workflow that works for you and the sort of speed at which things happen.
(c) You can use (if you want) proximity sensing so your mac will just unlock if you (or more precisely your [UNLOCKED!] watch/phone) is present.
Obviously the extent to which you want to use these different options depends on your circumstances and levels of paranoia. If you're afraid the police are coming after you, presumably you don't want any of the watch options since, I assume, if you're not fast enough, or are cuffed, the police can fiddle with your watch while it's still on your wrist and so still unlocked. If you're in an open office, you may not want the proximity options, etc.
I am guessing Apple has had this sort of thing in mind for a while. The main thing they've presumably had to work out is not the underlying tech (the author of MacID as one guy could do that, and I have a pretty good idea how he did it; there's no deep deep voodoo needed); the issue is the UI to present, how much configurability to present, and what defaults to use. Obviously a 3rd party app has this easier as the app is being sold to enthusiasts and fiddlers who don't mind having quite a few different options they can experiment with to find a workflow they like.