This is a really controversial topic flyingmind, but as I have shared elsewhere I actually agree with you.
Currently companies are in fact able to advertise on Steemit. They could open an account with a huge amount of Steem Power and start publishing product related content. If they are good at content marketing they would even find success doing that. We would all benefit from that because when those companies buy Steem the price of Steem will increase.
On the other hand,
I would indeed welcome direct advertisements on Steemit like for an example native ads. As you mentioned, that would really have a huge effect on the value of Steem. Steemit is already one of the few block chain based platforms with a current use. Advertisements would really increase the amount we are paid for participation and that would lure in even more users.
I agree that Steemit needs to be proactive now in order to fight off any future competitors.