"The slim mobile-centric device proves that HDDs may just yet have a spot in the mobile world"
HDDs shouldn't even exist in the mobile world. Their only advantage is capacity for dollar, but that's just for now. SSDs are faster, MUCH more reliable, shock-resistant (something HDDs can't mate with) and improve battery life. Also, capacity increases are more complicated in the HDD space, whereas flash doesn't seem to be hit so hard in that way.
"Q: May I Have That Shiny New Desktop PC With An SSD?
A: No, Sir, You May Not. "
GOOD. We, who build, fix and configure computers do that. If people buy new PCs with SSDs at low prices, things are only getting worse for us. SSDs provide great performance improvements, so leave that to us, not to companies such as Dell, HP and the rest.
Oh, and keeping PCIe SSDs out of the reach of such computer systems can only be great.
"The desktop PC market has been in the throes of a continued decline for several years, and it is easy to see why, with the apparent unwillingness to integrate SSDs into the newest desktop PCs. "
The fact that pre-built desktops don't feature SSD's by default is NOT the cause for a decline in desktop shipments. You've said it yourself: "the...market has been in...decline for several years" - SSDs have started to become somewhat affordable only recently. And yes, offering 512GB of flash for any cent over $100 is asking for too much for a lot of people. Back to what you were saying - you seem to confuse cause with effect here.
The fact that pre-built desktops don't come with SSDs is the effect of a lackluster interest in such computers, given what smartphones, phablets (increasingly popular), tablets (not so exciting anymore) and laptops (include hybrids here) are capable of.
Putting an SSD in a pre-built desktop is a synonym for "offering 2013's fashion in more colors" to many. Oh, and quite a ton of people react to new product generations (from iShit to Alienware-grade desktops) like this: "Oh, yeah, they improve every year, that's a given - I'll stick to my product until I really won't be able to do what I initially bought it for." Just like your mother's case: reading emails, using Skype & Office, watching movies & YouTube videos, listening to music, transferring photos, uploading them to NSA, cough, I mean Facebook and all that...guess what: it can be done with a Core 2 Duo, 4GB of RAM, an iGPU and a 250GB - 500GB HDD just fine. Hell, I know people who do that on Pentium D systems. Even $200 netbooks can do that with some issues in the video playback department here and there.