Disclaimer: I own zero cryptocurrencies, but have a decent knowledge of the space
Here's why I think Bitcoin will never go bust entirely, regardless of the day to day price: it's the area under the curve of POW energy expenditure. At this point, so much energy has been expended mining bitcoin that that in itself creates value. Even if the price crashes 100x it will always be worth something. And whatever that "something" is, no one can say yet. But I would wager that it's much higher than whatever the initial players bought in for (in terms of cash or energy spent mining). Sure plenty of other altcoins can come along in the future, but catching up with the level of work that has been done on the Bitcoin blockchain at this point seems impossible to ever do. It's simply a matter of compute power that is physically available.
But consider this... if the price were to go back to where it was just a year ago before the current mania (which is a common thing to happen when a financial bubble pops, after overcompensating to the downside) then it would be a 95% retracement from the highs (where, almost self-evidently, many people were buyers). And BTW a 95% retracement requires 20,000% gains to break even.
If you gave me a $1000 and I gave you $50 back, I think most people would say that I took all their money, even though technically they could still buy a couple pizzas or something.
So, again, it just depends on your definition of "full bust"
Bitcoins security relies on current hashing power and is completely indepent from past computations. If bitcoin falls to let's say $100 then major mining pools can do a 51% attack and make all the expended energy worthless.
Of course this ignores how hamstrung the technology is by the IRS treating it as property subject to capital gains tax. If I have to worry about paying tax anytime it is converted to fiat or exchanged for goods and services of real value, the less motivated I am to do so.
For Bitcoin to die out, every single person needs to decide that doesn't want it.
Since people want bitcoin primarily because they believe other people want bitcoin, if 90% of the userbase shift to monero or ether or some other coin, there's really no reason for the 10% to hang around. Zero is a very plausible price for bitcoin.
I agree that it can happen, like the exodus from Digg to reddit, but the brand is strong and it will have serious financial implications making it significantly harder than creating an account on the competitors product and continue as is.