then the altcoin has to be appreciating faster than Bitcoin, else there will be a pernicious downward spiral.
That doesn't make sense at all ...
I don't see why something has to be rising in order to speculate in something. Actually I'd rather like to speculate in something that got far less worth than it used to be than in something that got far more worth than it used to be. My profits depend on the %increase and my investment size, not on the actual value on the price tag.
That is a strong counter-point. If the weakness was perceived as a buying opportunity, then it is not a pernicious decline. I was actually thinking I should buy my own altcoin if it got too weak, yet I wouldn't want to be the only person buying it.
So really we come back to how many people strongly believe the features of the altcoin are going to make it popular for the masses? Features which Bitcoin can never implement. And whether the masses really adopt it, and don't just mine it then sell it for BTC.
If you can get the masses to mine it via CPU-only, then you've already got their first attention. Normal people hate to change too often, i.e. learn new software, wallet systems, etc..