All of those leaky pipes that they've not been replacing?
Haha! Yeah, but those will presumably be pipes that they haven't yet replaced either because they've figured that the cost of replacing them outweighs the benefit from replacing them, or because they intend to replace them but you can only do so at a certain rate because you only have so many trained staff to do that etc. A nationalised company would presumably make the same calculations, unless the Government changed the regulations to enforce more replacement (but the Government could do that anyway if it wanted to - no need to nationalise)
Future profits to the end of time?
Kinda. But allowing for the rate of interest. If - say - the rate of interest is 10% then a profit of £1000 next year is only worth roughly the same as a profit of just over £90 this year. As you extrapolate that further and further into the future then at some point the future profits become effectively worthless as far as estimating current value is concerned.
Unfortunately for Corbynite-Labour's buying-the-water companies ideas, the Government would have to do basically the same calculations because in order to pay
today to buy the companies, funded out of future profits, they would have to borrow the money today, and they pay interest until the future profits show up. (And before you tell me that the Government could pay a lower interest rate: If the Government borrows more money, that increases its total debt, which means people become slightly less willing to keep lending the Government money, which then puts upward pressure in the interest rate that the Government has to pay in the future on
all it's debt (not just the extra amount borrowed))
I've simplified a bit. There is an issue that a company's market value is based on a lot of estimating future profits because you can't know them for certain plus various other random factors probably including how investors feel on that day

, but overall, to a good approximation, if the Government wants to use future profits to buy a company, then the cost is likely to absorb future profits in their entirety, and therefore to cancel out the profits you expect to make.