How does Gandalf almost die in The Hobbit?

Question

Gandalf is one of the greatest - if not the greatest - wizard in Middle Earth. He manages to survive pretty much every encounter he has. But in The Hobbit (chapter 6 I believe) Tolkien writes:

Then Gandalf climbed to the top of his tree. The sudden splendour flashed from his wand like lightning, as he got ready to spring down from on high right among the spears of the goblins. That would have been the end of him, though he would probably have killed many of them as he came hurtling down like a thunderbolt. But he never leaped. Just at that moment the Lord of the Eagles swept down from above, seized him in his talons, and was gone.

What's really puzzling me is

That would have been the end of him

Gandalf manages to [spoilers!]

revive after a fight against a Balrog

and also manages a great deal of feats. Yet that passage states that Gandalf would have perished then and there had the eagle not come. My question is

Why does Tolkien write that "That would have been the end of him"?

Answer 1

This piece is a survival from the earlier editions of The Hobbit, before the idea of it's conception into Middle-earth. An extract from The History of the Hobbit describes Bladorthin (later renamed Gandalf) lighting the trees on fire and dropping a flash of light from his wand.

“And with ‘ya hoi’ the flames were under Bladorthin’s tree, and soon beneath the others. The bark caught fire, the lower branches crackled. [cancelled: Still they clung on – but] Then Bladorthin climbed to the top of his tree; the light [> sudden splendour] flashed from his wand like lightning, and he got ready to spring down right amid the spears of the goblins. [They scrambled away from > gave back >] That would have been the end of him, even though he might have [got >] killed many as [he] came down among them like a thunder bolt. But he never leaped. Just at that moment the lord of eagles swept above the scene, seized him in his talons and was gone.
_{The History of the Hobbit - Second Phase, VI: Wargs and Eagles}

This was part of the Second Phase of the writing of The Hobbit, when the story had gone from an interesting opening to a complete story.

While this sort of explains the "This is an part of an ancient version of the Hobbit, before it became part of Tolkien's Legendarium", we should not assume this is the only reason Gandalf could die.

Gandalf was clad as a man, and although his spirit could falter, he was capable of the death of his body like anyone else. Although he was a Maiar, and he had fought and defeated a Balrog later in his life, Gandalf was not all-powerful, he could have been overwhelmed by the goblins and have been killed had he jumped in the Hobbit. There were and overwhelming number of them and as Tolkien states in the excerpt, he would have killed many but in the end, it would've mean his end.

Answer 2

TL;DR: Despite his power, Gandalf is a mortal human just like anyone else.

It's important to remember that Gandalf - or an elf like Legolas - is just as vulnerable to mortal injury as the next guy. He may be a Maia in human form, but the important part is "in human form". He can be stabbed, crushed, or shot with arrows. Had he jumped down onto the goblins, the spears would have killed him as surely as anyone else.

Also, your note about his revival following the Balrog fight is based on a false premise. Gandalf himself had no part in his return - that was divine intervention and wasn't due to any special ability of Gandalf's or the other wizards.