Astral Fire/Umbral Ice timers are actually bad mechanics at a meta level.

[Ferrinus]

This is literally, and I cannot stress this enough, LITERALLY how every class in the game works.

Nope, wrong! Specifically, it is not how Black Mage itself works. It's strange that I have to tell you this, since the reason that BLM doesn't work this way is precisely what you're complaining about: AF/UI timers. The principle effect of AF/UI timers is that they force you to make compromises instead of executing your ideal rotation, because you're on a timer.

A simple example. The situation: you've just entered AF 3. Swiftcast and other cooldowns are down, but Triplecast is up. You have a Thundercloud proc, and Thunder's going to expire on your target in 10 seconds or so.

The plan: F4, F4, F4, F1, F4, Thundercloud Proc + Triplecast, F4, F4, Despair.
The reality: After your first F4, the boss targets you with a mechanic. You burn Triplecast in order to be able to keep casting. You throw F1 and begin to cast another F4, but you're targeted with another mechanic that forces you to run around for several seconds because AoEs chase you or something. You've already spent Triplecast; by the time you can stop running, you'll have only 5s left on your AF timer.

Well, you can use your Thundercloud, but then what? Do you cast another F1 to refresh your timer, and then proceed with F4s into Despair? Do you just cast F4->Despair and transition to Umbral straight from there? Maybe you throw in a Scathe while running around? Maybe you panic or get too greedy, end up casting two F4s, and then realize you don't have time for Despair or even B3 and just hit Transpose. Maybe you don't even realize you should Transpose so you drop Astral entirely, resetting your Polyglot timer.

If you had no timer, you'd be guaranteed the ability to cast your three F4s and another Despair and not have to make any of these decisions. This is why I said Black Mage has an aspirational rotation. There's an ideal thing you want to do, but the world will conspire to stop you, so you have to make compromises and substitutions based on your knowledge of the fight and your own job mechanics. If there were no timers, this wouldn't happen; BLM would actually be as simple and boring as you're pretending it is.