You've mixed up some things, I believe.

In the same was money distribution in the world is not uniform, in the same way Bitcoin distribution is not uniform. Some were luckier, some had money to invest, some have missed the train.

But this is is not related to mining. Bitcoin mining has became a business, indeed, average Joe cannot mine anymore. On the other hand this is good, helping blockchain safe against attacks and this also keeps the price high, so some of the "average Joe" can invest and gain (although, as said, this doesn't happen in many cases, but I'll get to this too).

And all this is is not related to decentralization as known for Bitcoin. Bitcoin decentralization is about how many full copies of the blockchain there are, it's about the full nodes and the number of mining pools. And these are fairly dispersed and decentralized.

Now about your concerns:

* Market manipulation is indeed bad, but isn't this concern out of greed "you want a piece too"? Bitcoin should be a coin, not necessarily an investment. Also the average Joe doesn't afford investing, this is the sad reality in the world. Bitcoin is not about easy money.

* Bitcoin mining is not dead. Just may not be a good business for average Joe. And we get back to the greed part.

* You are unhappy about the scams. Isn't like this everywhere in the world? The world is full of fake shops, of people trying to steal from you and so on. Here there's no difference, that's why one has to inform himself before trying this or that.