That really isn't so. Tolkien was not an immigrant, nor were his parents. His family was in England since the 18th C. and had been established in Birmingham since at least 1812. He was born in South Africa while his father was posted there as a bank manager.
The importance of Tolkien is that he was the only one of his time to see the true literary character and historical importance of Old English works, first Sir Gawain and the Green Knight and later Beowulf. Other scholars saw the monsters in Beowulf as childish elements set into the story as entertainment, but Tolkien saw them as integral to the narrative and to the historical events being recounted. He was also able to produce stories of his own that incorporated these elements as central to their themes, and for this reason he is called the father of modern high fantasy. Tolkien is the wellspring of everything from Dungeons & Dragons, Arena, and onward, and no matter how these differ in particulars from his universe, there is no more success in escaping his influence than there was in escaping the influence of Beethoven on 19th C. music.
It is in the raising of these old tales to high art and in the recognition of the fantastical elements as the very stuff of the narrative that all that followed him is dependent on Tolkien. The stories and the elves and the monsters would be there without him. But the weaving of these into a well-formed narrative is the fruit of his genius, and in this genre, his alone.