No, not at all. You make two rather large assumptions: that Government are prepared to spend the money up front to fund new trains; and that the money taken as profit by the ROSCOs would have been there to re-invest.the current system only encourages LOWER investment in the railway. I don't even know why I have to explain this; is it not obvious?
They may be prepared to do a little of that now, but back when Privatisation was first mooted the aim of Major's Government was to get away with as little spend as possible on the railways. That wonderful thing, hindsight, shows it was a flawed model - but it did save money for the Government of the time.
I wish I lived in your perfect world where economics is simple but I'm afraid I don't. Without the ROSCOs I think there would have been less investment in new trains than there is now, even allowing for their profit margins.