Spain has to be brought into war and the Straights of Gibraltar have to be closed to UK.
If Spain enters the war, Spain starves as its food imports are cut off from the US. Is Franco willing to trade the starvation of his people in order to assist the Germans in their war?
It is why he asked for so much OTL - the Germans simply did not have the ability to replace the US as a source of supplies (esp as the only alternate, the USSR, wouldn't be a good alternate to turn to)
Gigantic loses? Yes. Loses numbered in the millions? No. There is simply no way they will do that.
Also if Germany doesnt declare war on the US, even if the US still enters the war in Europe, the US will be forced to do a "Japan First" strategy, meaning that they will already be war weary when fighting Germany.
Considering that the US isn't capable of committing to its invasion across the Central Pacific until 1943 at the earliest, due to the need to construction the fleet and logistic train to support it, then the US will be committing forces to Europe simply because it has nowhere else to
send the majority of them, much less tanks/etc. The Italian campaigns and all would likely continue similarly to OTL. The US simply doesn't have the material ability to put enough men and ships into the Pacific to defeat the Japanese decisively in the same strategy as OTL.
And, if Downfall proves to have too many casualties (they will have a good estimate in the aftermath of Olympic), the US can simply blockade Japan and cut it off from the mainland, and let it wither on the vine if need be.
Defeating Germany would be quite a bit more important, as Germany is far more capable of posing a threat to the US. Don't forget that, during WW2, many in the US thought that the only reason the Japanese were capable of attacking Pearl Harbor was because the
Germans made it technologically feasible for them, as the Americans thought the Japanese were too backwards to be capable of launching such an attack on their own.