This is completely wrong, it's not pointless.
First, this can be used to easily swap out routers in a network without reconfiguring any clients or even incurring downtime. Without the intermediary gateway IP representation, this would mean you would either have to spoof the MAC on the second router or reconfigure all of the clients to point to the new gateway.
Second, ethernet addresses are a layer-2 construct and IP routes are a layer 3 construct. Your default gateway is a layer-3 route to 0.0.0.0/0. There are protocols for exchanging layer-3 routes like BGP/RIP/etc that should not have to know anything about the layer-2 addressing scheme to provide the next-hop address.
Third, routers still need to have an IP address on the subnet anyway to originate ICMP messages (e.g. TTL expired, MTU exceeded, etc).
Fourth, ARP is still necessary even for the router itself to know how to take incoming IP traffic from the outside and actually forward it to the appropriate device on the local network. Otherwise you would have to statically configure a mapping of local IP addresses to MAC addresses on the router.
So ARP is critical for separation of concerns between L2 and L3. We don't live in an ethernet-only world.
>excessive ARP starts becoming one of your biggest nightmares. It's especially bad on wifi.
Broadcast can become a nightmare. Excessive ARP is a drop in the bucket compared to other discovery crap that computers spew onto networks.
The pattern of most computers now is to communicate with the external world (from the LAN perspective) and not much else. So on a network of 1000 computers (an already excessively large broadcast domain), your ARP traffic is going to be a couple of thousand ARP messages every few hours. If this is taking down your WiFi network, you have much bigger problems considering all of those are about a modern webpage load of traffic.