With a smile!
Truly – for the needs of most small business, Office 365 services, including Azure AD (Active Directory) can provide all of the network services that have been provided traditionally by the on-premises office server.
There are office applications that are well known to not work well over the Internet or other WAN. Accounting programs are a widely used example. The transition by software companies to SaaS hosted solutions is widespread – it is highly likely that any software package that your company is currently using is now available as a cloud hosted service.
That leaves the networking services – DNS for name resolution, DHCP for automatic network configuration of computers attached to the network. Both of these services can be provided by the local Internet router or firewall present on the company network.
It follows, then, that the old on-premises server can be tossed following migration to cloud hosted services, including Office 365.