Paul Thurrot is the man in the know; he already has a pretty good idea of what is coming at this event:
https://www.thurrott.com/mobile/microsoft-surface/183481/what-to-expect-from-todays-microsoft-event
You have to remember that Microsoft's biggest customer for these devices is the enterprise. They must maintain backward compatibility and they have established the Surface Connect port as something they will support for the long-term (there are a lot of Surface docks out there hooked up to desktop setups). While I'm guessing that USB-C is on the horizon, it's simply not a priority for Microsoft and their core customers at present.
It's funny – the one company that has completely embraced USB-C is Apple, yet everyone I know with the current MacBook and MacBook Pro do nothing but complain about those ports and all the dongles they have to carry to plug-in even the most simple peripherals.