Nobody seems to even consider the possibility of replacing this modem.
Probably because ISP provided routers/modems use either coax cable or (in the case of DSL) telephone line inputs, so you are now having to add some hardware and corresponding difficulty for dubious gain; it might be worthwhile for educational or prototyping purposes but it is unlikely to be of practical value.
Further, most ISPs, particularly the large ones, aren't going to support you on this. They may not forbid it, but they very likely employ proprietary software in those devices that will make replacing them somewhere along a spectrum of difficult that ends with impossible. This may be a bit easier with smaller ISPs that use something closer to a stock router or modem you could buy yourself (but don't count on it).
If what you have already is something you bought yourself, then the last paragraph doesn't really apply, but I think now-a-days that is on the unusual side, which is why people who want to use their own routers attach them to the ISP provided one rather than trying to replace them. That is drop dead simple because it is just another network device and your ISP isn't going to care how many subnets or access points you want to deploy, they're just going to enforce whatever bandwidth usage limits apply under your contract.
However, the pi is not a good choice for this in any case unless you live alone and don't care much about connection speed. It is a low network throughput device; the ethernet and USB ports are all on one bus with a maximum throughput of at best, in theory, 280 Mbps (I've seen them hit ~30 MB/s writing to a USB drive, so the peak reality is close to this). If you are providing uplink access via such a device, that means the maximum volume is half that, since it must come in and go out the same bus, i.e., 140 Mbps. That's the equivalent of one normal wifi connection.
Also note the actual ethernet jack is only 100 Mbps.
The Pi 3's onboard wifi doesn't really save you from this (except in the sense that it may make the theoretical limit just mentioned more reachable), because it is limited to the speed of one such connection, 150 Mbps.
In other words, can you do this? Perhaps. Is it worthwhile for most people? Absolutely not, which is why you have not found much material related to it.