Photoshop ain't no "paint" program.
The dynamics of Photoshop vs PaintShop are very different.
Photoshop has INCREDIBLY advanced vector graphics capabilities, using paths and the subsequent mapping of effects to paths via layers.
Paint Shop did not introduce capabilities like this until recently and they're still not as advanced.
But.... also, people know how to use Photoshop, so even if PSP suddenly released a more powerful version, people would stick with Photoshop for now.
I haven't used PSP in about 2 years, so I can't speak to its relative improvements. I like Photoshop, but I've used it since 1996 or so and I still find new ways to do things every time I use it. One of the more complex programs I've ever used.
It could use a better interface- the tool boxes still bug me. I'd love to see animated toolbars like in 3dsmax.
Eric