I think the only possible worse case scenario on Dash premine is if the developers or whoever holds those premined coins starts selling the coins in order to take profit
There are no premined coins.
There were "fastmined" coins for a couple of hours during which many people mined.
The biggest holder however, was not a miner but a buyer, since even with the fastmine it was still cheaper to buy in markets than mine and there were tens of thousands on sale for several weeks for a few BTC.
That holder has since executed exactly your "worst case scenario" and sold nearly 80% of their holdings in OTC (over the counter) sales during the last couple of years. (Around 400,000 coins by most accounts I can gather).