What % of customers do you think give the OK and move on? When MB tells me I need something, I bring it my local guy who buys the parts at the dealership and replaces for a fraction of the cost, if the recommended action is actually needed. I would imagine most people are not doing that and say OK.
Other costs involved and agreed not so simplistic, but a basic understanding of how the plans work and more or less that is it. The bigger the plan the more assets under management the cheaper the ratios. Economies of scale do work here. Also, not saying one shouldn't keep a 401k but that rolling one 401k to another is a bad idea. Limited and expensive options in 401k as opposed to an ira with etf's or mutual funds.
Usually the provider or employer stick the fees of the plan into the expense ratios, you sometimes see expense ratios of 1.5% or higher. At .70%, it means the employer is paying some offset or the plan is really big. Although worst thing to do is roll one old plan into new plan, should roll out to IRA.