Halifax, NS, was doing that in the mid-'80s without the benefit of GPS or handy mobile apps. (Every bus stop/route had a phone extension, and major stops/terminals had talk boxes.) Granted, it wasn't a huge system, but it was hitting rocks with sticks and hollering over tin cans and string compared to what's available as a starting point today, and it worked. The idea that it should be a hard problem today, given GPS, vastly improved wireless communications and eleventy-seven different ways of figuring out where a user is (if entering a stop code seems like it might be too much work) is just plain silly. Or we've significantly redefined what "hard" means.
Or expectations have increased, both in terms of accuracy and in keeping costs low.
Consider that e.g. in London the old route indicators on the buses used odometers. Which worked for telling you were along the route a bus was. Most of the time. But didn't tell you how long it takes until the bus reaches the next stop other than on average, as it would tell you nothing about traffic etc.
Today London buses don't use just GPS to replace it either, because that's nowhere near good enough. See the "Tracking" section here [1].
When I said "worked", I meant worked, not approximated working or came lose enough not to have caused too many upsets. If expectations have grown to be higher than plus-ten-second accuracy (early wasn't tolerated), then we collectively need some sort of counselling/therapy.
You don't get that kind of accuracy in any system that has to deal with congested roads, or any dense major city. E.g. there's plenty of bus-stops near me where stops on the same route involves going around corners and traffic machines and stopping close enough that GPS would be insufficient to be guaranteed to tell the stops apart if you don't get a good enough fix on additional satellites quickly enough.
Even with all the extra data, and a lot number crunching, in London it's a challenge to get it to withing a few minutes during rush hour.
So, sure, if you're dealing with simple, small systems with little congestion, you can simplify.
