Because you can pay less taxes by maxing your 401k out. You can also access money in your 401k before age 59 1/2 by using the roth pipeline. If the plan you laid out will work and get up to normal retirement age, and then you have access to your 401k and it's too much money, then that means you worked for too long. If you max your 401k and utilize the roth pipeline, then you will be able to stop working sooner than your current plan.