Big whales can't cashout without dropping the price to single digits. The market isn't large enough to offset significant sell offs. You'd have to sell coins at a slow trickle for months and months to safely cashout.
But that doesn't mean that they can't cash out, right? Since we don't see price falling to single digits as you assume it means either that they are not massively cashing out or that they in fact do but it doesn't affect the price so much. If you ask me, some of them might decide to cash out at the end of the day, and some of them should have done exactly that already, for example, because of contingency or similar unforeseen events. It could also be that there are no big whales as such, apart from 1M bitcoins held by Satoshi himself, or they have simply lost their keys.
One of the greatest things about bitcoin is that you can't know it for a fact if there are the so-called whales or not. Surely there are those who are heavily invested in bitcoin. Needless to say they are also aware of bitcoin's liquidity levels that are still pretty low, so a massive sell-off will most likely push the price downward and this is, obviously, not something they are interested in, at least for now.