The benefits for the infrastructure accrue to citizens in the future, so its sensible to borrow money (at the very low rates that the government can obtain) and

The benefits for the infrastructure accrue to citizens in the future, so its sensible to borrow money (at the very low rates that the government can obtain) and pay for the benefits as they are enjoyed. If the additional cost of the interest makes the project not worthwhile, then this means the project shouldn't be undertaken, not that it should be paid for upfront.