>>How can apps be fragmented on Windows Phones?

That's not what I said. ;-)

>>Did you actually look at the chart I linked to? It has nothing to do with apps, but shows how the manufacturers orphaned the hardware by not allowing/supporting updates to the OS.

If your phone has an old OS, does that mean it won't run the latest apps? Otherwise why does it matter unless a big security hole appears?

As for the chart: IMHO these phone makers know from past experience that consumers hate messing with phone internals. As smart phones emerged from geekdom to the wider community, perhaps the goal is to make newer models as accessible as possible. IOW get a new one rather than messing with the internals. If hardware and pricing goes the way the experts say, it's a rational approach IMHO.