The ancient machines were more reliable. I worked at a McDonalds for about 5 years in the 1980s and cannot recall one day when the Taylor machines didn't work.

The old machines had higher training and labor costs though, and over thousands of stores this was an issue. Presumably the newer machines are overall less expensive, even accounting for the lost sales on days where they are offline, and the associated repair costs. They may also be necessary for the newer ice cream products. In my day we only had vanilla soft-serve (cones or sundaes) and shakes with three flavors.

McDonald's isn't in business to keep Taylor repair techs employed. If it was costing them more than the alternatives, they would make a change.