I see it more as existing holders making an unbreakable pledge to hold it for a certain amount of time. It breaks the prisoner's dilemma that most coins are held hostage by. That alone is enough to at least attract new investors.
How can making a coin "unsellable" attract investors ?
You mean knowing that everyone else is locked in as well makes it more attractive for me to be 'locked in' ?
If demand isn't there the price will plummet anyway, regardless of what the supply is. There is some precedent for this model though. Right now the US fed has trillions of dollars locked up on their balance sheet which they've kept "off market" by offering a higher rate of interest on excess reserves to banks than they'd get on the open market.
So that has supported the value of the dollar despite a near 5-fold increase in its supply. The difference here is that the dollar 'engineers' already have a significant amount of pre-existing demand t work with, so they can use supply as a lever. If you don't have the demand in the first place then controlling supply alone is about as effective as playing tug of war with an ant.