I'm A Banana
Potassium-Rich
Whether or not a person, spell effect, etc. is "good" (or "evil" or "chaotic" or "lawful") can be objectively determined, that much is true.
What's not objective is any particular criteria by which you can determine what thoughts/actions/beliefs will place/expel you from that particular status, how many/much of them are required, etc. The idea of the alignments being "objective" is only true insofar as determining whether or not you've attained a particular alignment at a given time. What makes you have, or lose, that alignment is still entirely subjective, hence why we still have alignment debates over forty years after D&D's release.
So if someone asserts that no "good"-aligned deity would ever allow good-aligned mortals to go to an afterlife of eternal suffering just because they didn't believe in any particular deity, well...that's not a matter of opinion, not an inherent conflict with regards to (in)actions-versus-alignment.
I didn't say objective, I said not dependent on a fictional deity. It's not objective - a DM or an individual player largely determines what is acceptable within the bounds of a given alignment. That's VERY subjective, but within the world, the fictional characters act according to these alignments as interpreted by the people playing at the table.
But alignment is a bit of a higher order of abstraction from the problem.
If I want to play a character in a heroic fantasy game who sees Nice People suffering at the hands of others, and so fights those who inflict suffering (lets call them the Forces of Badness) to bring joy to the land and the people, I'm kind of forced to conclude that in FR, deities such as Lathander and Mystara and Chauntea are at least complicit with if not aligned with the Forces of Badness, because they sit idle while Nice People suffer. They even BENEFIT from the setup. And if I, a heroic character fighting to save Nice People from Badness, think that this makes them unworthy of my worship, because I want to save all Nice People everywhere and none of them apparently do, my soul becomes one of the Nice People who the Forces of Badness inflict suffering upon.
I am unable to save the Nice People. I'm not even able to save myself.
That's not a world of heroic fantasy, that's a world with levels of "the bad guys rule the world" that not even Dark Sun has.