x86 is hardly any less proprietary than PowerPC or SPARC. You've got Intel and AMD at the helm. VIA walked the plank ages ago.
Apple ditched PowerPC because Apple's market share was so fucking low that the only company compiling for PowerPC was Adobe. The decision to drop PowerPC had to do with market share and cost, not the architecture itself.
Yes? No? I think this is a misunderstanding of the motivations behind Apple's PowerPC switch. (Source: I wrote PowerPC Mac apps at the time and was in the room at WWDC when Apple announced the switch.)
The PowerPC market was a bit wider than that. Microsoft had Office on PowerPC, Adobe had their suite, and there was a smattering of other apps.
At the time, the future of PowerPC had looked pretty bright. Microsoft's Xbox, Sony's PS3, and Nintendo's Wii were all switching to PowerPC. Within a span of several months, the community was looking at a majority of gaming hardware being PowerPC based. PowerPC was going to be in very high demand, which would mean great things for the Mac PowerPC platform. Far from "the only company compiling for PowerPC was Adobe", Microsoft was buying Power Mac G5 boxes for their dev kits and they were porting Windows to the PowerPC for the Xbox. And in the end, Microsoft, Sony, and Nintendo combined shipped several hundred million units based on the PowerPC (With Nintendo still shipping the Wii U with PowerPC today.)
So why did Apple leave the PowerPC?
At the time, laptop sales were on the rise, but Apple's laptop CPUs were not designed by IBM, they were designed by Motorola. IBM's PowerPC G5 was suitable for the Xbox 360 and desktop machines, but it ran far too hot to go into laptops. This left Motorola with their G4 CPU. And let me tell you, Motorola probably had very smart people working for them, but their execution was incompetent. The G4 had a 133 mhz system bus (which was slow even for the time), and ran very hot (but still cooler than the G5), and worst of all, was much slower than Intel's Pentium M.
Meanwhile the Pentium M was doing very well. It was faster than the G4, more power efficient than the G4, and it actually had a modern chipset and bus. Switching to the Pentium M was a no brainer.
There was speculation that Apple was trying to get IBM to make a mobile G5, but they were never able to get the power consumption down. When Microsoft and Sony entered onto the scene, IBM's interest shifted to getting the PowerPC into larger form factors, and Apple just didn't ship enough units in laptops to balance out the R&D demand that Microsoft and Sony created.
Motorola in the meantime with the G4 just kept sucking. There was a new architecture that was basically a modern architecture for the G4 that did eventually end up shipping, but by then Apple was just done with PowerPC.
Intel provided a stability the AIM (Apple, IBM, Motorola) alliance just didn't provide, with a quality chip. PowerPC did end up scaling, but there simply wasn't the same demand for PowerPC machines at the time to make it scale well enough.
So were people not actually writing code for PowerPC? No, lot's of people were. I'd actually guess that after Apple left PowerPC, the number of PowerPC developers continued to rise. And with the Xbox 360, Sony PS3, and the Nintendo Wii/Wii U continuing to get new games, there are still a lot of PowerPC developers out there.