In short, McDonald's continues to use these frequently malfunctioning ice cream machines due to their long-standing exclusive partnership with Taylor. The cost of repairs fall on the local franchise owners, not McD Corp.
McDonalds has Walmart level bargaining powers with vendors. The current setup, where a machine breaks and you have to call a specific technician for insane rates is not an accident. McDonalds could change that tomorrow if they felt like it. No business relationship at McDonald's scale will ever be tilted away from their benefit.
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.