But at first brush my feeling is that the US can provide the Pacific power much more safely and close to as effectively as a UK/Russian investment. And the UK and Russia are better situated to use their IPCs against Germany/Italy.
The problem with letting USA fight it out alone vs. Japan is that it gets very difficult to outproduce them. The idea with keeping India is to keep Japan honest, somewhere around 40-45 IPCs instead of those impossible 50-55 IPCs you see if Japan runs rampant. Then USA can mount a credible threat without dumping all their IPCs into the Pacific. My impression of the game is that USA is in the best position to bring Italy down. It’s hard for UK to send their fleet down into the Mediterranean since they usually have to keep pressure on Karelia in the early game. And due to the Luftwaffe, splitting up the UK fleet is also difficult, leaving it to the Yanks to destroy the italian fleet.
When Japan is hindered enough, you can then refocus all of your production to Europe for the final push against Germany (the early game you put 20 IPCs in Europe and ca 15 in India). Also keeping India means UK doesn’t drop down too low in production. South Africa as an IC is nice but it helps mostly to keep the African IPCs, hard to threaten Japan from down there.
As for Russia, they should pull out their expensive units, fighters or tanks, as soon as India is secure and bring them back to their home front. Fighters are great here since they can be based in India and take part in your counterattacks vs. Caucasus. The plan is that once Moscow is in danger, you should be able to withdraw them!