TL;DR> MAC addresses are a low level component of an Ethernet network (and some other similar standards, such as WiFi). They allow a device to communicate with a machine on the local physical network (LAN), and cannot be routed across the Internet - because physical hardware might in theory be plugged in anywhere in the world.
By contrast, IP addresses cover the whole internet, and routers use them to figure out where to send data even if it needs multiple hops to reach its destination – but they aren't helpful in interfacing with the physical hardware on your local network.
If we ever found a better standard than Ethernet, it might not use MAC addresses but IP traffic from the internet could still flow across it, even if other people on the internet had never heard of it.
If we ever found a better standard than IP (for example IPv6 if all the IPv4 addresses ran out), most Ethernet hardware could carry the new kind of traffic without modification – and a simple software/firmware update would fix most of th