Do you think that in my case, this can work? If you think in another perspective, this can be considered a "Reward Removal" system, since I am giving "free power ups" to the player and removing after 5 game sessions. The player would be able to still play, and even win, but he will certainly miss the power ups.
Well, when you take something away it can irritate people. It's like raising taxes- it's always a lot easier to lower them.
It's better to dangle something good in exchange for payment rather than taking it away.
Slowing things down (which I thought you may have meant) probably wouldn't work in tower defense like it works in empire building games.
Here's a better option: Maybe give them the ability to rewind, so if they are premium, they don't have to repeat the full level if they fail. That's something that saves the player time, but doesn't keep them out of your game.
You could give them a small taste of it by trickling in occasional items that allow it (in a limited sense).