I think one of two things could happen, possibly both.
1. They will place these on routes that are known to have more people actually paying for the premium seats compared to using their points or miles.
2. They’re also adding premium seats on up to 100 other aircraft including United mainline. Even if people are using points to upgrade, if there’s a better chance that they’ll be able to get first class they’ll be more likely to use United in general. A lot of frequent flyers have multiple loyalty programs so if they think there’s a better chance for an upgrade then they’ll be more likely to stick with that carrier. It’s similar to hotel points. I’m a Hilton and IHG member. I try to stick with Hilton because usually their hotels are a little nicer, and I also usually get upgraded to a better room more often than IHG.
Usually the money they gain from keeping or having a more frequent customer is worth more than the money lost giving away a free upgrade.