so much this:

what would happen if you removed every mystic coin that wasn't intended to be used from every player (ie, the ones that are being kept PURELY as an investment)?

NOTHING.

the people hoarding aren't selling and the are NEVER going to sell because the price keeps increasing, it would be insane to sell!

functionally those coins may as well not exist, they are never going to get sold because the demand FAR outstrips the natural supply, and therefore they keep rising in value, why sell something that you KNOW will get more valuable?

in fact, by ignoring it, ANet have made the problem even worse, the MOMENT the price of coins drops, the hoarders will panic sell, utterly flooding the market and driving the prices down even more.

Adding more coin sinks won't work, because whatever would be valuable enough to convince hoarders to sell would also be something everyone else would want, making the prices rise further, and therefore making it even more lucrative to just hoard

ANet need to understand that the ones hoarding the coins are not the ones who will be tempted by coin sinks, because the raw gold value is FAR more important to them.

if ANet want to use coin sinks to deal with the number of hoarded coins, they need to get the coins out of the hands of hoarders and into the hands of those who will use coin sinks

the only way to do that is to convince hoarders to sell, and the only way to do THAT is to make them think that their investment will go down in value. and how do you do that? increase the supply of coins SIGNIFICANTLY, make them panic sell and make the coins worthless, let the ones who respond to coin sinks throw them away, and THEN consider where you want the price of coins to be.