Well, criddell's claim is that there are a lot of people who really prefer to use Apple's payment system rather than a third-party one, such that apps that don't support Apple Pay will lose their business.
If those people really exist in large quantities (and I can believe they do!), the developer is losing business by not providing Apply Pay as an option. But that'd just lead every single customer to use Apple Pay, which is not what the developer wants. So you need to somehow give the people an incentive to choose your version, unless they really think that going through Apple is giving them a lot of value. Differential pricing seems like the most obvious way of doing that.
> Maaaaybe competition would force you to drop prices but a lot of consumer software isn't fungible and doesn't really compete on price.
That seems implausible. If you think that software prices don't matter at all, such that lowering prices does not increases sale, it'd follow that raising prices doesn't lower sales either. In that case, why hasn't the developer already raised prices?