Grayscale is a custodian for clients that are institutions and serious investors. It acts something like an exchange outside of the crypto world offering shares of Trusts and these shares are later traded by these investors if they want to in OTC exchange where trillions are traded.
Any amount of BTC stockpilled does not make a difference to the network. The worst that can happen is Grayscale to get hacked somehow and the wallets to be compromised but I'm sure there would only be cold wallets so it will have to be an inside job. Meaning there probably won't be any chance of these Bitcoins to be lost. So a repeat of MtGox has close to 0% chance.

I would be interested if there is another danger for this operation, except that the investors don't actually own the Bitcoins but a represantation of them in stock form.