G3100 and E3200 have an internal switching capability of 3Gbps, a routing capability of 1200Mbps, I believe.
G3100 has a WAN MoCA 2.5 chip within, which allows it to accept 2.5Gbps on coax, but MoCA is hardly a sustainable standard for the future. G3100 only has gigabit WAN port. So considering these constraints, G3100 isn't able to move ahead with a future PON upgrade.
There are multigigabit capable routers on the 5G side. I would imagine the engineering would combine the Fios and 5G Router product lines together in the future.