I highly doubt that an individual who commits a serious crime over the internet without masking their IP won't be investigated.
Wifi routers are insecure and often can't be flashed with more transparant/secure software, so in practice anyone within 100m with a directional antenna can connect to your wifi.
People let guests on wifi, and students tend to share internet connections.
Some devices randomize their MAC address by default (Apple stuff I think) and MAC addresses are trivially spoofed.
If I'm on your wifi router I can probably spoof the MAC addresses of any and all of your devices, and I definitely can if you've ever connected to an AP managed by me.
Lastly, ISP's can spoof everything and fabricate logs with ease.
So no, technically there is no proveable link between IP address and person or device. Perhaps legally but that has no bearing on the technical reality.