these stickers and effects would have been fine in a separate app or being able to fully turn off the new graphical stuff. Snapchat is only used by certain groups and messages is used by all groups from kids to millennial to baby boomers which have the same quantity of smartphones as the millennials. There is a reason facebook stayed vs myspace, customization that can't be controlled to something standard by the visiting users killed it.
Facebook vs. myspace, with respect to customization and visiting users, is irrelevant here. Those sites are one to many, in a detached (write now and maybe someone will read later) manner. Messages is user-to-user, and immediate. And all that you have to do to not be bothered by emojis, stickers, and silly effects, is to: a) not use them yourself (it's not like Messages is going to continually prompt you that you're not using enough stickers), and b) request those with whom you are communicating to not use them. If they refuse to comply, shout obscenities at them and block their number so you won't be bothered by them or their stickers in the future. Really, this is a social issue, not a technological one. The app is fine. Apple is providing what (at least some - it appears many) users want. I don't expect I'll be using stickers much, but it doesn't bother me to have them available. If you find yourself getting stickers,
sent to you by people you know and have elected to communicate with via person-to-person messaging, your problem is with those people, not with Apple. Insisting that, because you don't care for stickers and such, nobody should have access to them, is... kind of annoying.
Apple appears, this WWDC more than ever, to be providing features that users and developers have long wanted - more ways of funding apps (subscriptions), Siri for 3rd-party apps, CloudKit for non-MacAppStore apps, ability to hide stock apps on iOS, piles of improvements to the watchOS UI (including much faster launch times), the new filesystem, tabs in windows on macOS, single-sign-on and dark mode for tvOS, and... expanded message features (and 3rd party extensions) for Messages.