So, you might not be aware that the 10Gbe interfaces on the Macs that have them do support 2.5Gb speeds as well. It's just that the chipset Apple chose for this is capable of going all the way to 10Gbe where many more consumer oriented devices that support mGig support 2.5Gb only, for example.
Use the 10 Gigabit Ethernet (RJ-45) port on your iMac, iMac Pro, Mac mini, or Mac Pro for data rates up to 10 Gbit/s.
support.apple.com
As far as multi gig internet goes, FIA in any country tops out at 1Gbps. Multi-gig will arrive (initially as you point out it will probably be delivered as consumer grade 2.5Gb interfaces but that's still a ways out) but it's still a bit of a con game.
Unless you have a truly dedicated connection (fiber) that "multi gig" connection (or any other connection for that matter) is shared with all the other users on the node or span. Additionally, you are constrained by every other link in the connection all the way through to your local point of presence (POP), the head-end connection and eventually the speed of the server you are accessing that you want to get multi gig from.
Amazon, MSFT, Google, etc... as big as they are will not allow you to pull from their connections at those speeds. They do traffic shaping that limits the speed from their servers and nodes to make sure that a user with a super fat connection doesn't fully consume the circuit or server and starve out all of the other users.