Investment with btcs don't work, people on the internet are only looking for your money (to steal it). Any moment this can happen, be prepared!
I am definitely agreeing with you that there is no better investment than simply holding your coins safely offline. But the point is that a lot people here want to put their coins to work in the way that they will increase in quantity. Exactly this is what makes people put themselves into big risks. Especially because of the fact that most of the investment options are either a scam from the start, or they will become one at a later stage when their site or service doesn't bring in a decent amount of profit anymore. That's why every investment option must be treated as such at the very beginning.
The issue of holding Bitcoin and investing it is such a two way thing. Most people now see BTC has an asset rather than money, if you don't invest your money there is no way it is going to increase in value and even inflation is going to rob you of the your money, just imagine what you can buy with $100 in 2009 and what you can buy with it today. I don't buy into the idea of holding BTC but another way to look at it is that Bitcoin is deflationary and the value is expected to increase has more people are expose to it but this theory is only going to work provided there is continue demand for it in the near future.