I agree that progress feedback for users is important, how exactly would it work? The example you show is for scanning a found set of records from the first record to the last record — possible the most common type of loop in FileMaker — and it's easy enough to infer progress by the place of the current record within the found set. But what if you made the loop move backwards through the found set? What if you scanned the found set in random order? What if you're doing a "while" loop rather than a "for" loop? I don't expect the Loop script step to infer what the progress is for any of these cases, or even to infer which case is the case.
As you mentioned, developers have already built their own progress bars. Perhaps FileMaker could provide an OS-native version we could use instead of our own approximations, but I don't think FileMaker finds that very compelling. From FileMaker's perspective, I imagine that making the same software platform work on Mac, Windows, and iOS is much easier when you roll your own rather than using each platform's own native functionality. FileMaker may be less interested in providing us something we can already do for ourselves. Is there a variation of the feature that we can't already hack together?
I'd certainly like to see a built in Filemaker native progress bar object. But we need a way to hide and show it via script (along with other fields/objects as well).
I understand the issues (I think) with regards to what FMI may find compelling to do. However, in their quest to make the product better for the user/developer class, a built in progress bar has become common for a lot of software products.
As to how exactly it would work ... I'm sure the engineers would have a more exact answer but I figure it would react to the same tenets as does a sort progress bar now. It counts the number of records that are in play and counts down without regard to the contingencies you cited. I'm pretty sure they are cross platform. One thing I don't know is would a built in system have a smaller overhead than what we do now.
I'm not sure what the threshold is for FMI acting on requests but I imagine it's factored by who is doing the requesting to some extent. So if that is true the folks who have the juice likely roll their own and it will become mute.