The Starter 401(k) very likely is going to be the darling of fintech. The narrow focus of the design will allow them to create a simplified administrative structure from document signing through recordkeeping, investment, and ultimately distributions with very low overhead. Existing recordkeeping systems are designed to be all things to all plans and have an enormous amount of overhead to anticipate the myriad of ways plans can go off the rails. Take things like employer contributions, allocations, vesting and forfeitures off the table and the technology infrastructure shrinks dramatically.
Expect the cost of set up and recurring services to be exceptionally low, and any tax benefits to the employer for starting and operating a plan are an added selling point.
The biggest difference with using IRAs for an employer is the Starter 401(k) will have a single payroll feed to a single provider, and will plan level reporting available.
Starter 401(k)'s will be available nationwide versus state plans. Starter 401(k)'s will have a much broader market available to them over state plans. Keep in mind that almost half of all businesses (and almost of of them are small businesses) do not have a retirement plan.
There's more, and there were advocates who are prepared to implement Starter 401(k)'s that lobbied Congress to make them available.