I don't really think it would be worthless. If less people would use it, the price would go even faster because there would be even a smaller amount to be used, so a smaller available supply. I'm not sure which one drives the price higher though: people having it but not spending it or people spending it more than ever before?
Yes if the Bitcoin transaction not happens means there is no value to use Bitcoin, and if all bitcoin users stop to use bitcoin then the bitcoin may disappear. But it will never happen, in future also don't think like this. Daily countless bitcoins transactions are happening. And a lot of people are earning money from this Bitcoin.
Yes ,bitcoin price will collapse as the investors are gone .
I don't think also that it might happen it just an speculation .More possibility that comes on my mind is if bitcoins dump in the future. A lot of people earning on bitcoins now ,Yes but the question how long it will take if bitcoin mining stops and end circulating .We cant predict and assure if bitcoins will be always on top . That's why i am thinking some profitable activity and enjoying the advantage of bitcoins now .As much as possible i don't only save bitcoins ,i also save real money .