That's a hard one to answer, because I think a lot of people are going to jump ship at the 1k mark (like we saw last year). Basically I see it as being like a wave. We'll hit ATH, people will jump ship and lower the price. It will climb back up, hopefully past the ATH the next time, and the cycle will continue.
I think much depends on the speed of a rise towards $1000. If it was a grind upwards over months and months then I guess many would unload out of sheer boredom and relief once it got there.
If it was looking explosive then good old greed would take over and many would sit tight and wait to see where it ended up.
It's the opposite. If it went up slowly people would dump on the way up. If it shot up, people would try to guess what the top is and dump in the hopes that it'll drop by 50% again and they can buy in for 2x as many coins.
It just depends on the person and when they purchased Bitcoin and it could also depend on their reasons for purchasing it. Then there are tax ramifications for cashing out that might cause some to want to hold for longer. I know that I personally wasn't in a place to sell last year when we reached $1000 because I would have had to pay short term capital gains taxes. Of course, there are those that try to avoid taxation (buying gift cards or whatever else they do) but I happen to try my best to be a law abiding citizen. Shocking I know.
It really is not all that simple and some people will want to buy and hold until they get to $100,000 if it really does get there.
It's an interesting thing to think about for sure. We all have our ideas on what's going to happen, but we could also all be wrong, :p. Cryptos have proven to be a wild beast in the past.