The Marine Corps, Navy and Army indeed gained dozens of little unsinkable aircraft carriers through their Island hopping campaign. When you think about the Islands that were invaded instead of bypassed, they were to gain airfields to permit our forces to close on the Japanese mainland: Henderson Field and its Cactus Airforce, the Islands attacked in the Marianas, Iwo Jima, right up to Okinowa. The necessity of holding the airfield at Midway from Japanese conquest is another example.
There would have been no such islands to "hop" from in the Atlantic, except maybe the Canary Islands, which are a heck of a lot further from Normandy then the British Isles.
And remember, landing a numerically superior force to wipe out an isolated garrison on a small island is a very different proposition from landing on the mainland of a continent and facing (for the first time in combat - there would have been no Operation Torch, Operation Husky or Operation Baytown) the experienced Nazi Forces.
And also, even with the Island Hopping, we never had to actually invade the Japanese mainland. The proposition was so daunting, that the military ordered half a million body bags. I have no doubt at that point in the war we would have succeeeded, but how many of us on this forum would never have been born because our fathers would have lost there lives in that invasion? I don't even want to even contemplate the losses of American, British and Australian/New Zealand lives if we had to do it. I am just thankful that Truman had the guts to do what was necessary to same half a million Allied and probably 3-5 million Japanese lives.