The two-sided argument is quite clear and straightforward. If you took ANY business and regulated one side of its income stream, it will have to do several things. One one side it will have to reduce its expenses. Reduce costs, reduce investment plans, reduce returns to investors.
On the other side of the coin, it will have to increase its income from other sources. Typically, around the world, this means charging the consumer more.
Your supposition is that by reducing MTR it will automatically reduce retail prices. This is simply not true. If anything, the opposite happens. Reduce income from one source and we'll have to find it from another.