I would love to replace these "routers" with a normal computer. The thing is these computers would need special ports for either phone lines or fiber optic connections, as well as built-in modems. I've never seen a computer with this sort of hardware built into it. Even on dedicated network cards I only ever see ethernet ports, nothing compatible with whatever it is my ISP is using (SFP?). Decades ago in the dial up days I used to be able to buy modems separately but not anymore, and I'm not even sure what sort of hardware components are needed for a fiber connection...
My ISP gives me a box that terminates the fiber and has ethernet on the other side. They also rent and sell routers that are configured to handle the pppoe and vlan settings needed for the WAN interface to this box. Plenty of routers can do this, and a dedicated Linux box like you are proposing should work, or you can throw a cheap managed switch in between if not. The hardest part is knowing what settings are needed (e.g. I had to call my ISP to ask for the pppoe password).
DSL standalone termination is still widely available, as are standalone DOCSIS cable modems.
If you're on DSL, the DLink DM200 has an integrated adsl/vdsl modem that's supported by OpenWRT, and the platform is quite powerful. You'll need a second device for wifi though but that suits my use case.
If you want a dedicated OpenWRT device Mikrotik would be my suggestion.
