Not to cast too much shade on this idea as it’s clearly had some effort out into it but the key scaling factor for Netflix is that they primarily just distribute content to people. Cost of production of new material is then amortised across the user base.

This isn’t true for your service on the face of it as you will still face the problem of hiring and employing good designers. In particular to offer a 24 hour turn around with no client meetings they’d need to be extremely gifted and a little clairvoyant! Not to mention that as your business scales your staff will likely scale at the same rate. This is why design agencies typically cost more to businesses than employing in house designers as they are competing over the same staff with similar costs so to get some profit they need to cost more. Obviously the advantage offered is not needing to keep staff employed when there design needs are lower.