I doubt the hacker will deplete those tokens because of the current fees. The hacker will spend more on that case.

But seriously, MEW is totally safe. We are the ones who aren't making it safe due to some human error or possibly got caught on a scam trap. Treat our keys like how we protect our exchange account, bank account, or any financial accounts. For newbies, there are lots of safety measures being shared on the internet. Take time to browse those.