Can you imagine what he might have become as a composer if not forced to flee Russia?
This is one of the great what-ifs for the whole of Russian culture of that period, not only Rachmaninov! Medtner, for example, fled abroad too. Stravinsky and Prokofiev should be mentioned too. I think that when a culture is divorced from its roots, something changes and something is lost... although something is gained too. Whether there's a "winner" in this game of win and loss is hard to say. Non-USSR Russia might have formed some kind of counterweight to the disintegration of tonality that happened in Central Europe? Of course this happened to a certain extent anyhow with the USSR, Shostakovich etc (serial music being effectively banned in the USSR) but Russian music was wretchedly undermined by its soviet masters. I somehow doubt Rachmaninov would, or could, have continued writing "romantic" music, or even that he would have wanted to?
I often think that Stravinsky's personality would have caused him to work outside Russia anyhow, and that his life was just as much a predisposition to working in the USA and Southern Europe anyhow as it was an unwillingness/inability to "go back".