Privatisation increasing investment means one of two things, either the investors are getting ripped off or they're going to get a profitable return on their investment. Assuming no government would admit to the first and no investor would intentionally invest in the first, let's assume the second is the plan. In which case, if the investment will yield profits, there's no reason the government shouldn't make the investment themselves so that the public get the profits (as well as the benefits from increased investment at the start).
That's such a ridiculous argument though.
Privatisation increasing investment means one of two things, either the investors are getting ripped off or they're going to get a profitable return on their investment. Assuming no government would admit to the first and no investor would intentionally invest in the first, let's assume the second is the plan. In which case, if the investment will yield profits, there's no reason the government shouldn't make the investment themselves so that the public get the profits (as well as the benefits from increased investment at the start).