Perhaps, but if I'm the DM and I see the player rolling 7 damage dice instead of 5 I'm going to want to know why....
Ok, so set the expectation that players state when they’re upcasting their spells. NBD.
And this assumes, of course, that the player's goal is just to fry that one goblin. Maybe she's picked that one to target because there's 5 others around it. Or maybe she's really trying to light the curtains on fire to block view into this room for a few moments. Player doesn't say any of this in "I cast fireball centered on this goblin".
I mean, if there are 5 other goblins in the blast radius, they’ll make saves too. If there are curtains in the area, they will be ignited, as per the rules text of the spell. I don’t understand what your argument is here.
Nor does the difficulty change depending on their goal. I cast fireball for fun. I can’t it to kill one goblin of 10. And I cast it to kill all 10 goblins. All different goals from a strictly GAA. All have the same approach as well...
First of all, it’s not unusual for different actions to have the same difficulty. Second of all, there is never any uncertainty with fireball, unless someone tries to Counterspell it with a lower level spell slot than you’re casting it at, and in that case you do have to make a check to resolve that uncertainty. Otherwise, you say you’re casting it, it works without a check. That is precisely how it
should function under G&A.
If there happen to be goblins in the area, there is uncertainty as to whether or not they will take the full force of the detonation, so they have to make saving throws.
I don’t see how casting a fireball fits in GAA. Square peg round hole is what it looks like to me
It works exactly the same way any other action works. The DM describes the environment. The players describe what they want to do. The DM decides what happens, often relying on the roll of a die to determine the results, then narrates the results of the character’s actions.
Now, that process of the DM deciding what happens, often relying on the roll of a die to determine the results? What exactly that looks like depends on the actions the players take. Casting a spell happens to be an action that has a lot of very specific rules about how to resolve it. Most other actions are much more reliant on the DM’s judgment. But both fit just fine into the same action resolution framework.