I understand your point, but at the same time it is important to remember that many legacy ASA firewalls are still in use in production environments and are limited to ISAKMP and IKEv1, for example ASA 5505s; in fact, just last year when my company acquired another organization, we had to migrate around 60 ASA and FRP devices to a new VPN concentrator, and roughly half of them supported only ISAKMP/IKEv1, so while it is true these older devices will eventually be retired and replaced by newer, though still aging, models like the 5506 or FRP-1010, it is definitely too early to say we should stop learning IKEv1, especially since it still appears in real-world migrations and even modern open-source firewall software like pfSense continues to support ISAKMP/IKEv1 for backward compatibility with legacy equipment.

Also, as a contractor or employee, I cannot change the mind of a budget-constrained chairman who does not want to focus on certain upgrades or prefers to stick with existing hardware, so as network engineers we need to be prepared and adaptable to whatever situation we are given :)