Are there any specific areas of SharePoint 2016 Enterprise that really require SSL ?
Yes, if you are using the ADFS authentication then you need the SSL for all your Web applications which configured for ADFS.
If you want to Configure Exchange task synchronization in SharePoint Server 2013(It’s very important that you have Secure Sockets Layer (SSL) running for your SharePoint web application, this is required for security reasons)
Other than that i think this is optional and you have to decide where to use and where not.
Unless you don't care about security and replay attacks? No.
TLS is used for the usual purposes for HTTPS sites (nothing SharePoint specific here).
TLS is also used to protect the OAuth cookies when communicating between SharePoint and Office Online as well as SharePoint Addins.