There a two things competing with the power plant:
1) Another plant, like the nuclear plant. Is the benefit worth the loss of a big boost in hammers?
2) Processes: Does the cost of the plant pay for itself compared to the boost you would get from just running processes?
The thing about the end game is, while the turns take a long time there are few of them left in the game. So late game buildings have to pay for themselves very quickly, if they don't then they weren't worth building in the first place. Yes the new power plant is much stronger than the old one...but that's what you have to do to be worth building at that point in the game.
For example, I have always thought Nuclear Plants were really good. CrazyG challenged me on that, so in one of my recent games I actually did the math on how much benefit I got from the plant once the game ended (and this an actual game end, not theoretical). In that particular example, I only got 3 turns where the nuclear plant provided a net benefit (aka gave more hammers than its initial cost). I probably would have been better off working a process in that game. Now I don't think every game will be like that, I probably got the nuclear plant later than I should have....but it showcases how even a mighty +50% bonus can be weak when your dealing with the quickly ending late game.