Here's a PSA, and a short answer.
PSA: Please don't trade things that you don't understand. That's rule #1. Not that you're doing that. You're trying to educate yourself, I can see that clearly. This is directed at anyone else that comes across this post.
Short Answer: Think of buying VXX like buying insurance on the stock market. Like other forms of insurance, it loses money over the long run if you're the one buying it and makes you money over the long run if you're the one selling it, but... Sometimes bad things happen, and then insurance payouts happen, and as a result sometimes insurance companies go broke.
If you're dead set on trading it, first checkout the CBOE site for VIX futures and then the prospectus. If you can't make sense of that stuff, well, see the PSA.
Adding in response to OP's comment:
My answer wasn't meant to come across that way. It was meant to be a warning. The group of VIX-based ETFs and ETNs are some of the most complicated financial products that exist (which individual investors can easily trade).
You asked how/why a speculative financial instrument even exists if its value is predetermined? At first you might roll your eyes at this one, but this is important: it's value is not predetermined. Saying that over some unspecified period of time something should trend towards 0 is very different than knowing on any one date exactly what that thing will be worth. Why? Path dependency. The path matters. If a seer told you that you would die a rich man that may please you. If the seer then told you that you would be poor until the week before you died and then get rich very quickly, well, you may not like that as much. And what if the seer said that all of this would happen over the next month? Then you're really not happy!
To tie this back into your specific question:
- If you short VXX, depending on how much you short, you yourself can go bankrupt depending on how much of it you short and how much it goes up. I'm not aware of an actual limit on how high VXX can go (if there is one i'd be interested to hear that).
- A big thing to consider is the utility(think of this like the value to you) of the cashflows from your trade in different states of the world. If there's a trade where you make money when the world overall is a happier place and you lose money when the world overall is a scarier place, that is going to be looked at differently than the opposite, assuming the probabilities and dollar amounts are similar. This can be tricky to grasp for some but let's say there's a 50% probability the economy is chugging along just fine a year from now and a 50% probability we're in a recession. If I told you that you could either choose (A) to receive +$10k in the event the economy is fine and lose $10k in the event it's not, or (B) lose $10k in the event the economy is fine and receive $10k in a recession, which would you choose if you had to choose one? You should choose (B). Why? Well, it's much harder for you to get $10k if we're in a recession and you're more likely to be losing money elsewhere and we humans generally like to not end up in the street and/or hungry-- we want to smooth out our wealth over time, we dislike volatility. That $10k when things are going badly is worth more to you.
Do all of those points make sense? And yes, this is pretty complex.
To wrap it all up if you agree with the above:
Since VXX does tend to go down over time, it does tend to go up when the market goes down (and most people own the market and things like the market), and because cross-asset correlations have been generally trending higher over time (so it's "harder" to diversify than in the past), it may make sense for someone that understands exactly what they're doing to short some amount of it(or do some similar trade) in the context of a broader portfolio, but probably not as much of it as they would have been short in years past.