I've not read the book, but I would think from an interest perspective you've got to make the dragons vulnerable somehow.
I agree The Hobbit crossbow is a little lame and uncreative, but using normal spears it appears you can hurt the dragons as happened in Merene.
The dragons need to be vulnerable and I'm okay if one of them gets killed. Hopefully its done in an interesting (white walker magic?) and plausible way.
We're well beyond the books now in terms of story, so it seems that the crossbow is a loose story idea conveyed to the showrunners by GRRM, or a creation of their own. But we do know from the show that Drogon was injured by the spears in the fighting pits of Meereen.
As far as backstory about the original dragons? In the books GRRM talked about how Aegon the conqueror came to Westeros with his three dragons, his being Balerion the Black Dread. It was the skull of Balerion that Qyburn used for his demonstration to Cersei. Following Aegon's conquest the dragons became smaller, weaker and eventually died off. They weren't immortal by any stretch of the imagination, and it's been well established that the dragons can die.
As far as comparisons with the Hobbit? I can see the parallels one would draw with the movie. FWIW, in the Hobbit book the black arrow was fired by Bard using a bow, not the large crossbow as depicted in the film. Nevertheless, in both stories you're essential dealing with an alternate medieval era along with similar medieval levels of technology. A crossbow or a ballista (those date as far back as being used by the Roman Empire) is the only weapon of that era that could fire a projectile that might kill a dragon. The only other weapon that might be some sort of threat to a dragon would be a trebuchet or catapult. But that's really more of an indirect fire weapon akin to artillery, rather than a direct fire weapon like the catapult that can fire an aimed bolt or spear. And that direct fire weapon is the one you'd probably need to take on a dragon.
I don't like to speculate on what may happen to the dragons, but I think it's very likely that they may not all survive to the end of the series.