Scrum, for one agile methodology, is not at all about replacing people with cheaper people. It's all about getting a tighter feedback loop on smaller chunks of work. It's meant to deal with specification drift, scope drift, and people talking past one another about the customers' needs.
Having two weeks or four weeks to deliver a minimum viable product, then increments of the same for new features to be added, is not something one should ask of a team of plug-in commodities. It takes a team some time to work together to even be able to estimate properly what they can deliver in that amount of time. Once the team is built, it should be left in place as much as possible.
The practice of specifying every little thing up front can be handy if those specifications are done well and won't change. If the customer's needs or which needs take priority over other start to drift, then getting that feedback every few weeks becomes really valuable.