People keep repeating this and it's so obnoxiously false. Earnestly worshipping something is not simply making a few prayers and sacrificing a few coins. Doing just those things is not earnest worship. I can't see how anyone would consider that earnest worship. "Oh yeah I tossed some coins in the well and said a couple Hail Mary's, I'm good for the next month." That is the definition of lip service. More than that, every god has different requirements, there are codes, whole systems of beliefs and rules that their worshippers need to follow.
The return isn't that great either. "Eternal paradise"....well every god offers that. It's not exactly like its hard to come by. There are so many FR gods each with their own private paradise, and I hate to tangent into real life, but the Wall and FR gods in general is a very judeo-christian conceptualization of polytheism, you ignore the costs in the now in favor of the rewards in the then. In classic polytheism, worship was less important than being a respectable person. Gods recognized and rewarded heroic, honorable, noble deeds for worthy causes regardless of if you prayed to them.
That's not true. You had to make sacrifices and practice rituals. In general, no one denied the existence of the gods in the society that was deemed respectable. In Greek society, the gods were extremely respected and powerful. They had rituals and holidays to honor the gods. You were expected practice those rituals and offer sacrifices and prayers or you would suffer the gods displeasure. It was the same in the Norse religions.
A lot of what you claim with rewarding noble and honorable deeds was after the fact. Oh, so and so was so brave, he must have been inspired by such and such god. He will be rewarded in the afterlife. Certain behaviors were deemed admirable by the gods and to be rewarded by the gods of the Greeks, not by other gods because everyone was assumed to be worshiping the Greek Gods.
Judeo-Christians did not start religious exclusivity at all. That game was played for ages. The only difference was pantheons rather than monotheistic entities. You picked the appropriate god to pray to for the thing you wanted.
The one idea that isn't well represented in the Forgotten Realms is the idea of a single realm where all the gods live is the place where the dead go to reside. In Greek myth, you didn't go to each gods realm. There was but one great realm where the Gods lived and one Underworld ruled by Hades. The Gods were part of a pantheon that the Greek people worshiped as a whole. That is not well developed in modern D&D polytheism.
Then again D&D is has its own religious structure. That structure doesn't have much verisimilitude if the faithless in essence get a free pass for being faithless based on their actions. That renders the gods impotent. No deity with actual power would allow such a thing. At their core gods are selfish and power hungry, that is why they demand worship and manipulate the affairs of men. They give with the expectation they will receive rather than give freely. In the D&D world, you either become immortal, become a god, choose one to worship so you have an appropriate afterlife, or suffer the consequences of faithlessness.