They are, but in my opinion five years to show something is pushing it. Particularly with only 38 players on the list. My personal view is that tall guys really have to be at a point by the end of the second year where they could play AFL footy if required. They don't necessarily have to be a superstar in their 2nd or 3rd year, just contribute otherwise they are just clogging the list.The big fellas do usually get quite a bit of time to develop, Laycock at Essendon and even Hille come to mind. The big fellas 6 foot 5 and over usually do need that time as well.
Clubs have held onto tall blokes for a long time you're right. Some do it more than others. You named Essendon players, the Bulldogs have also done it with Cameron Wight (7 years) and Wayde Skipper (9 years). In my view, that's being too patient. There's a point where you have to cut your losses and admit they aren't going to make it.
Players who get drafted with the mentality that they have an eternity to develop in the VFL have the wrong idea. A tall is a legitimate development/project player for two years, and by year three they need to be read for senior footy, maybe not play it, but at least be ready to.
Just my opinion.