The 77D is probably more about marketing than technology. The more choices Canon give customers the more likely it is that one of them will fit their needs and price range. That's the theory anyway. It also causes confusion about what to bay and what the differences are. Too many options con't always make life easier for customers.
The main "upgrade" of the 77D is the use of the new DIGIC 7 processing chip. This appears to allow them to offer a slightly higher ISO range, presumably due to slightly better noise reduction algorithms. It may also slightly improve AF tracking performance. However these are likely to me marginal gains. Measurably better in lab tests but probably not really noticeable to user who isn't looking for them.
As it is now I think the 80D has a longer battery life, a faster shutter, higher viewfinder magification, a faster shooting rate (both JPEG and RAW), better weather sealing and a headphone jack for monitoring audio when shooting video. The 80D also has an 8 way control bad, the 77D has a 4 way. The 77D has bluetooth, is slightly cheaper and has a larger buffer, plus the DIGIC 7 advantages.
I'm sure the 90D will have all the additional features of the 77D (mainly a DIGIC 7 processore), plus more. Canon's main goal in fife is to get you to upgrade in small step. i.e. for you to buy a new camera every year or two. They are all good cameras, each one slightly better then the previous one. If you buy the 77D now, Canon probably hope that you'll move to a 90D when they release one!
The EOS 77D is $ 899 and should be shipping by the end of March (http://www.adorama.com/ica77d.html?kbid=12417
The EOS 80D is $1099 and ships right away (with $44 in store credit) (http://www.adorama.com/ica80d.html?kbid=12417
So I wouldn't say the 77D is an 80D killer. It's an option with fewer features in general, but with a few things the 80D doesn't have. There's still a choice there!