Why should these whales get a safer way to invest? If they don't trust you they can go away.
They don't only have to trust that I mean well, they also have to trust that I don't accidentally lose the wallet, or get robbed. I can have the best intentions in the world and still lose your coins.
Also if they keep coins locally and the site ruins badly one week, only the investors who trust you will get burned. These lucky whales from the 1 cent days will simply run and leave us to burn.
I'm not sure you understand how it works, given that you're saying this. Even with the ability to keep the bulk of your coins locally, I'll never let an investor risk coins that I don't have physical access to, so there won't be an option to "run and leave us to burn", if by that you mean they leave owing the site coins.
The risk as I see it is that someone deposits 1000 BTC, claims to have another 49k BTC locally, and so I add 500 BTC to the max profit. They then quickly lose that 1000 BTC and don't deposit any of the 49k BTC to top up their on-site investment, and so I have to liquidate their position and reduce the max profit by 500 BTC. Their (relatively) small deposit had a large effect on the max profit, both up and down, which looks bad.