I wouldn't say there's any "extra" traffic travelling through that firewall.
If you have 5,000 requests inbound, and you send an even 1,000 requests to each server, then that's no more requests being serviced by the firewall than if you sent 5,000 requests so the one server, or if you put the firewall behind the F5 (all 5,000 requests still need to pass through that firewall at some point, otherwise they're not on a "private" network at all).
But it is true that the firewall is a single point of failure, but if you're dipping into the budget to fork out to purchase a single F5, well then that F5 becomes a single point of failure as well.
If you're out to configure a fully redundant system, you need two F5's in an active/passive HA cluster, and then you would have two firewalls, also in an active/passive HA cluster.
They may be depicted by a single graphic in the F5's documentation, but that's because it's just showing the logical appearance of the firewall (there's one device serving all the requests), not the physical setup (two devices, one of them in HA standby).
Another reason to put your load balancer behind your edge firewall is because your load balancer may not be web hardened by default (perhaps it has vulnerabilities in its management interfaces, maybe it comes default permit-all, who knows). By putting it behind the firewall and only poking holes for your publically required ports, you run a vastly lower risk of a vulnerable load balancer being exposed to the internet.