You may have cause-and-effect reversed here. If tech companies focus their innovations on mobile and treat desktops as second-class citizens, then of course, you're going to have a drop in consumer interest and sales in desktop PCs. IIRC, PCs were experiencing a slump in sales back in the late 90s when Apple introduced the iMac and the whole anti-beige, Internet-era makeover thing caught on. Mac and PC sales took off again for many years after.
If that were true then the PC desktop market wouldn't have seen sales drop at all, as Dell, HP, and others are offering more new models than ever before.
It's not simply a matter of "If you build them they will sell." as you claim. People don't want desktops as much anymore. They want portable. That's why we saw tablet sales expand as desktop went down.
