for those who have dared to take the risk of investing in 2015, I thought at the moment even when they sell, they can get quite a large profit because of the huge change in value
Exactly, you are right people who invested in the year 2015 are already in huge profit but i hope maybe those people are already exit at the time of bull run in 2017. For the people, who are still holding the coins are in huge profit at the current values.
I believe that in bitcoin with each exponential price rise such as 2013, and 2017, there are more people who are stocking away BTC and just putting them in cold storage. Maybe not all of their stash, but a decent portion. Of course some of those who bought before mid- 2017 (which would have been lower than $2k) would have sold some or all of their stash at various points after late 2017 to present.
But in the end, with the passage of time, BTC is likely gaining more new adherents than it is losing, even if it seems difficult to measure. There are also people like Jack Dorsey who are just buying into BTC and not even being very secretive about it, which is going to cause more and more difficulties to push BTC prices down further or to hold them down, even if there are temporary achievements to get BTC prices down.
In that regard, if bears, whales and manipulators cannot push the price down anymore because there is at least a certain quantity of folks who are not selling, then they are only left with UP momentum that has its own kind of contagion, even if getting to UP momentum can take many months to play out or even to figure out with any kind of level of confidence.