Here's an interesting little thought experiment:
Why did JRRT make Frodo the protagonist of TLOTR instead of Bilbo?
It would seem unsurprising for JRRT to mirror his beloved 'Beowulf'. Beowulf is in two parts: Beowulf's fight against Grendel and Mother as a young man, and Beowulf's fight against the Dragon as an old man.
Why not Bilbo's quest to Erebor as a young Hobbit, and Bilbo's quest of the Ring as an old Hobbit?
Bilbo might be a better choice by Providence as Ring-bearer than Frodo? He is more experienced. He has more Estel. He is more willing. There is really no evidence that he would find it harder to resist the Ring than Frodo, or that he would be less likely to give it up and throw it into the fire?
Compare and contrast Bilbo and Frodo. Why did JRRT not just mirror 'Beowulf' and make Bilbo the protagonist of both quests? What is it about the differences between Bilbo and Frodo which JRRT thought were important? Why create a new protagonist for TLOTR?