I never said replace, just augment. You may need fewer, but you still need people to monitor, deploy and orchestrate these different services. At the end of the day the extra cost is going toward hiring engineers at the hosters side for managed services but benefiting from their scale.
Either way your going to have to pay for skilled operational engineers.
Competent, experienced staff to run the core of your business has a cost. You can replace ops with managed services no more than I can replace devs with cheap, untrained labor in Asia.