During the Ignite session titled “Run Microsoft Exchange Hybrid for the long haul”, Microsoft has hinted at potential solutions to the problem that many customers face after migrating to Exchange Online.
One of the frustrating discoveries for a lot of customers is that moving your email to Office 365 doesn't necessarily mean you can say goodbye to your on-premises Exchange servers. Even if you've used a hybrid configuration to get through the migration, and then removed the hybrid config at the end, you'll still need to maintain an Exchange server on-premises for management purposes if you continue to use directory synchronization (which many customers do).
To be clear, it's not the hybrid that creates the dependency of an on-premises server, it's directory synchronization. Directory sync makes your on-premises Active Directory the source of authority for your directory information (users, groups, etc). It's super convenient, providing same sign-on or single sign-on access to Office 365 services. But the catch is, you need to manage the user attributes (including email attributes) on-premises. And the only supported way to manage email attributes is using the Exchange management tools, which means you need an Exchange server.
There are some benefits to keeping an on-premises server though. Using it as a local SMTP service is handy, and if you leave the hybrid configuration in place it also provides you with the flexibility to off-board from Exchange Online in future if the need arises.
Still, customers who want to reduce their on-premises server footprint will always want the option to ditch Exchange. I've often thought that a lightweight “management appliance” might be able to do the job, something that can co-exist on the same server running the directory synchronization service as well. How feasible that idea is, I don't know, but its encouraging to see that Microsoft is actively pursuing a solution to such a regular complaint.
Update: at Ignite 2017 Microsoft revealed they are working on a hybrid connect to solve this problem.