Why do Evolutionary Design or Iterative Design or Incremental Design? Everyone who has not worked in an evolutionary manner asks this? My answer, if you think the system you designed is NOT GOING TO CHANGE EVER then sure you can do design once and deploy once and you are done, move on to next project. But tell me one project you have been on, that does not have any changes in requirements, changes in technology, changes in look and feel etc after it was deployed. So if every project changes after it was deployed, why live in the fallacy that nothing is going to change ever.
Instead, since you know requirements change, why not get better at managing change, which mean get better at Evolutionary Design or Iterative Design or Incremental Design, so that you are ready for the next requirement change that comes along.