I just say Boromir would provide the more interesting conflict in the long run. The attack by the Orcs would have split the group anyway so why not keep the guy that wants to use the Ring as tool against Sauron?
He also wasnt as shallow as the other characters, he had more sensible underlying motivations, apart from saving the world/lets help Frodo thing. Boromir wanted to please his father, make the Stuarts of Gondor count for something and directly safe his home with the atleast halfway sensible plan of using the Ring against Sauron.
And by "shallowness of the orcs" i mean Tolkien did send Mooks and Henchmen. A distant lingering treat replaced the graspable and tangible one that Boromirs desire for the ring presented.
And yes Tolkien was a masterfull planner, among other things. I just think storywise offing Boromir right there was a wrong decission.
Also Frodo and Sam didnt have all the means. They didnt know where to go (making Smeagol useful) and the needed later camouflage taken from the orks which killed each other (a bit to) conveniently in the guardtower. That and Galadriel can foresee the Future so she got them what they needed in Lorien. Well except a way back to safety once the ring was destroyed. Tolkien had to pull that damn "deus ex machina" with the eagles to get a happy ending. Galadriel thought, in my opinion would have willingly sacrificed Frodo and Sam to off Sauron.
All i want to say that Fellowship is kind of bland, Towers and Return got better with more character development, even in the books, without taking anything out of the story. Still In comparsion to todays much more refined approach to fantasy storytelling the books lack. Which i can forgive them, in my mind they are still awesome and with right the archetype for modern fantasy.
My point remains though, not including the new/made up stuff in the Hobbit would have made it bland and uninterresting. We care about the character because they are actual characters and they have stuff to work through. On screen, in a way we can sympatise with them.
The original story works in the Books, because its suited for the medium but for the silverscreen its not.
Also the new stuff serves as distraction from the Ring. We all very well know its a THE McGuffin. We also all know the End of the Story. The ending wouldnt provide anything meaningful to someone who saw LOTR.
Instead we get to go on an eventful awesome and twisting ride which on itsel is fulfilling.