It's cheaper to run a high speed service than a low speed one. You can transport more people with the same number of staff and trains because it runs faster. The solution isn't to run an artificially cheaper low speed service along side, it's to run the high speed service in a sane way.
Is it really? Because that claim goes against my intuition so if it's true I would be happy to get more details! But what you say doesn't quite make sense to me, sorry if I seem pedantic: transporting people faster is not the same as transporting more people. You transport more people per unit time, but not necessarily in total. I also don't see how faster trains need less staff. When you say it's cheaper, do you also take into account investment cost, or do you neglect those and just mean operating costs?
I'll express my last bit of disagreement with your reasoning and then I'll probably leave this argumentation, but I will read if you choose to respond. This is not what cost means, you are basically saying that your gut tells you it should be cheaper without any supportive arguments. If e.g. the train requires more energy to run faster, that alone could make it more costly, even if it has a higher capacity. Since neither one of us seems to have idea of the actual costs of running trains, I don't think we'll get anywhere with this!