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.
This realization seems to indicate there can only be one coin that survives, unless there is some reason to hold the coin other than its relative rate of appreciation.
Back in the early 90s, there were probably tens of thousands of individual bulletin board services. Most towns had several, and there were about half a dozen national ones in the US alone.
Each one was its own walled garden, with little to no communication between them.
In the late 90s as flat-rate ISPs began to enter the market, all the walled gardens got steamrollered by the Internet's network effect.
A tiny, statistically-insignificant minority of geeks prefer the walled gardens, while the rest of the population like having a single email address that works everywere.
It will be the same with cryptocurrencies. There will be one winner that achieves mainstream adoption, and a few toys currencies and that are kept alive by the few people who enjoy running them as a hobby and using them as testnets for new features.
That doesn't make sense either. Just because better tec replaces good one doesn't suggest we will all use just the better tec and not lets say 2 better ones equally ...
A better example for arguing your way of thinking would be on BluRay vs HD DVD. Here your argumentation would work, but thats wrong either. Just take a look at DVD vs BluRay vs HD DVD and you see the dilemma.
It will never be just BluRay. Even if we wouldn't use DVDs anymore there would be a new format out by then so it will be Blu vs new disk. The networkeffect killed HD, that's true, but it could only do that because it was cheaper to use just one than 2 practical identical tecs. First of all is BTC to f.e. PPC not similiar in that kind of way (POS rewards) and it is actually better to use more than one crypto.
Any feature that is so heavily demanded will be added to Bitcoin, because that is the most economically valuable solution.
Dream on. It would be impossible to f.e. add at least some POS blocks to Bitcoin although it would be economically better.