The official word is that Teams is a single client experience to handle many functions within Office 365, one of which is Skype for Business (SfB). Much like how you can access Word and Excel via Teams, SfB is still a stand-alone product, but the features of which are available in Teams.
“There is no end of support date at this time” for Skype for Business
Having said that, as the functionality transfers into Teams, along with user adoption, then logic would show that Skype for Business (online) certainly seems to have its days numbered. Microsoft has outlined its roadmap for transitioning Skype for Business (SfB) to Teams, suggesting you:
Optimise your current SfB environment for teams;
Pilot Teams alongside SfB;
Upgrade and Coexistence of both
Driving User Adoption
They also make it clear that existing tenants of SfB On-Premises will continue to be supported with Skype for Business Server 2019 due by the end of 2018.
So from my point of view, I wouldn’t start rolling out Skype for Business in my organization, without trialing Teams first. If I already had Skype for Business, then I would be working down the path to transition. Teams has great functionality beyond communication and you will benefit overall for doing so. Check out the FAQ’s page here for more information.