There will always be another new coin… each cycle will convince less people, but people don’t want stable, that want growth, and sticking with major coins limits that. Thus I see a good few more cycles to come, a few dozen coins pop up, ~3 get good attention, 1-2 crash of which die and the remaining on stays basically just as a public share to be traded, not a mined currency. Something like half the people who did well want a repeat of the results of rapid growth or mining, so they attract a few others each and a few dozen new cryptocurrenices pop up…
Long run I agree, the chance of each new currency being stable long term drop with each on the market, but just as long after people thought the few chain based fast food giants where stable, bam subway or dominos shows up late, and is a huge hit due to some unique feature or unfair advantage.
As for drop in card sales, its economical to mine on cards long after payback times are not reasonable to buy more. Ie as long as power use is below earnings you mine, you only buy cards if you are getting earnings high enough to pay back the purchase cost and your estimation of risk. Also mining is a much lower portion of all card sales than you may expect, which is why you don’t see many dedicated mining hardware. Stuff like a software and drivers are cheaper to roll out, and adding features for mining rarely reduces utility to anybody else (eg >8 card multiprocessing or a peak efficiency preset, not just regular and some factory OC).
I’d go one step further and say because used cards (especially mining cards) have low used value, plus you typically totally loose the cost of risers, dedicated mining boards and whatever low end CPU you had, miners have a vested interest in new things to mine. It always seems like a waste of power to me though, as other non-gold or USD backed currencies maintain agreed value without wasting work.