Sometimes its no fault of anybody, its just the length of the queue. E.g. green card renewals, simple, used to be done in 6 months or less. Now it is assumed that it will take 24 months. This i90 has no burden of proof or stuff, its just simple pay fee & get new card. So, sometimes applicant thinks rightfully they are stuck in there for ages, and USCIS has no saying in hiring new staff or implementing new laws or changes.

Maybe someone who knows queue theory could explain this, but if circumstances have driven the length from 6 to 24 months, doesn't that mean it's heading upwards to unlimited lengths?

In (basic) queue theory, at equilibrium (average input rate = average output rate), the length of the queue is proportional to the variance in the input rate. Make that what you want :D