Just imagine what happens if the futures curve remains constant for a few months. e.g. near month = 30, far month = 35. Every time you roll you are realizing a loss of $5.
It's not exactly like this. You don't lose the $5 difference, you lose the difference between the price that you rolled over at (In your example $35) and the price you close out later when the contract nears expiration, which should be close to the spot price.
When closing out the position later, it can be at any price, can be at $20 or it can be at $35. The contango effect makes it very difficult to make money on the long side this way since you are facing a natural downward pressure for the contracts you are holding to converge downwards towards the spot price.
So essentially, you buy high and settle low, rinse & repeat & you keep losing. The difference between the contract you are moving from and the one you are rolling over to is of no material importance because the price you will settle at is what matters to conclude your P/L for that cycle.
