It's easy to let your guard down once you've survived an initial challenge but, unfortunately, there are many more issues you'll encounter. You endure the sleepless nights of infancy and the tantrums associated with toddlerhood and, lo and behold, they start driving and dating and you discover the previous challenge was easy by comparison.
That's not to say you should continually stress about the future -- that doesn't benefit anyone -- but planning and anticipating future needs can help you better adapt to the inevitabilities ahead. Take for example, your retirement. If those children are your sole focus, you can be left on a financially barren island once they're out of the house, out of college and married. Assuming that your life will be easier in the future is a good way to guarantee that it'll be tougher. It pays to plan.
Life as the parents of multiples can be intensely rewarding, but there's no denying that it's challenging and difficult, as well. Knowing what those challenges are can help you better handle them.