That's not true. The miners have a large capital investment in mining equipment which can be used for other purposes (or they rent, in which case the same reasoning holds). Proof of stake participants have their capital in the system's currency. The PoW are less sensitive to price because they lose less capital for the same change in price as compare to PoS participants.
Decred has ASICs available, and (hardware) miners are usually the largest fixed cost for miners, so Decred miners do have a fairly large interest in keeping the price high at least in the medium-term of their hardware’s lifetime. Unless other cryptos of similar size are also using BLAKE-256 as their hash algorithm.
> Proof of stake participants have their capital in the system's currency.
Their capital can also be moved to other proof-of-stake coins with complete ease, I'd argue that's far less friction than repurposing your mining ASIC's for other coins on a whim. An ASIC is useless unless it runs the same algorithm, which in Decred's case is basically no other coins.
> The PoW are less sensitive to price
Yes, and as such more likely to ride the out fluctuations rather than create a self-sustaining wave of those who secure the network selling out all their holdings and moving elsewhere.
This and paulgb's reply are both good points, but I think that we can do a very simple analysis to explain most of what is going on.
Suppose that miners only mine and stakers only stake. Maybe we could justify why that is the case, but for simplicity we can start with a model where that's just taken as given.
Also suppose that participants do only two things with their mining and staking rewards. They take profit by selling, or reinvest.
Miner's reinvest by selling the token and buying more equipment or funding operations.
Stakers reinvest by staking more token without selling.
All else being equal between the two except for the fact that one is mining and the other staking, the miners will sell more of the currency.
Under this simple model, it's easy to see why miners are more sensitive to price in the short term and are more likely to sell. Of course this model isn't perfect, but I think it's close enough to explain the structure of the debate.
I don't know how decred works, but typically there are consensus level barriers to this to mitigate "nothing at stake" attacks. For example you may need to lock the tokens for weeks or months to use them as stake, and it may be impossible to trustlessly transfer ownership of locked tokens.