Long sentences make us keep more state about the current thing we are parsing in our head, as later portions of the sentence may cause reinterpretation of earlier portions.

What you may be seeing is less that people are becoming incapable of reading long-form prose, but more that prose itself has shifted to using shorter sentences and simpler structures, at least in contexts that have to do with reporting.

I don't find it surprising that people that are exposed less to longer, more complex sentence structures will have more problems parsing them correctly. In any event, it's stylistic choice, and one that some may find pleasing, but it does make the work less approachable (those used to longer sentence structures should have no problem with shorter ones, but the opposite is not necessarily true).