So the salient point is the browser will prompt the user for each signing. So this means the user would have to confirm via prompt to the browser for each posting action such as voting, posting and editing a comment, etc..
That is a significant irritant¹ to the users as compared to perhaps just doing better security. There is a trade-off.
For the more infrequent cases of signing for money transfers where a browser prompt would not be an irritant¹, do you think we could trust sandboxing the money signing private key? I would think hackers would then have the incentive to target the browser and the user's computer. But I guess it is no worse than having the user enter their signing key for money actions every time, because a hacker can still try to intercept these. Really for money signing, it seems hardware wallets are the way to go for larger balances.
Note for those hoped for microtransactions ecommerce in the future where money transfers signing will be frequent, then a prompt from the browser for each one might be an irritant. A better solution may be to have smaller balances for that private key and just use the better security strategy.
¹ Not only an irritant but users may become so accustomed to dismissing these prompts as quickly as possible, that they maybe can be fooled into acknowledging them when the hacker has issued one.