Who cares if they pay attention to 429s? Your load balancer is giving them the boot, and your expensive backend resources aren't being wasted. They can make requests until the cows come home; they're not getting anything until they slow down.

If you're rate-limiting by IP, well... some entire countries have only a handful (or one) externally visible IP.

And some of the bad bots are snowshoeing across many many IPs (and fabricating UAs). How is that load balancer going to help?

For IPv4 sure, but have you heard of our Lord and Savior IPv6?

My local monopoly hasn't. Maybe in 20 years.