Why would you pay $1350 for a crop sensor camera when a FF sensor is $1500? (for example)
Depends on the rest of the features. If the 7D with its 18MP, 19pt AF, 8fps, etc were $1350, and the 5D2 with its 21MP, 9pt AF, 3fps, etc were $1500, which would you choose?
Those two may be 'old tech' by now, but they pretty much perfectly sum up the differences that aren't just sensor-related.
If FF gets cheaper (which it is, gradually, bring on the D600 to spur competition!), and mirrorless comes in from the bottom-end, maybe we will see the end of the APS-C DSLR. Or maybe we won't, there'll be just more choice.
Imagine the scenario in a few years:
At the low end of the scale, I could get a basic aps-c 1400D my-first-dslr, or a half-decent m4/3 or aps-c mirrorless for the same price. Maybe even the latest iToy with a m4/3-ish sensor and glass with 'zeiss' written around the edges.
Midrange, you get more FPS in your aps-c dslr. Or you get a my-first-FF dslr. Or a top-of-the-range mirrorless aps-c. Or a my-first-FF mirrorless, just a sensor in a matchbox with a lensmount and not much else.
Up a step again, and you're pitching the best aps-c dslr, maybe a 7D3 with 10fps and f/8, against a decent FF dslr, or a top-of-the-range FF mirrorless.
More money again, the specialties like M10 or M11 (where price doesn't really reflect features of other brands), the 1DXmk3, D6, 16fps sports models with 93pt af, where phase-AF is still better than any live-view (although the gap is closing), even the my-first-Medium-Format Pentax 645Dmk3 (basic edition).
Just because mirrorless is getting bigger doesn't mean it'll kill off everything else. Go back 100 years and everything was large-format, bellows and huge plates. Then add in the 6x7 (6x45, 6x9) 120-film foldup cameras, they didn't kill LF. Then add in 35mm rangefinders, and the 35mm SLR, they killed nothing. Add in 35mm or aps film P&Ses. Add in digital P&S, DSLR, and then mirrorless.
Nothing has killed off anything, you can still get large-format, you can still buy a Horseman or Arca-Swiss 4x5 brand new, medium format, film SLRs, film rangefinders, they're all still around. They may have 'had their day', and us film shooters may be a dying breed. But there's still a market for 'specialty' items (look at Holga's growing market, for example).
Maybe things will change with the dgital market, more R&D has to go into todays cameras than earlier types. But i don't think so. I'm putting money on still being able to buy an aps-c dslr on this day in 2022...