If you don't know what a 51% attack is well, it's a situation in which one of the nodes in the Bitcoin network has more than 51% hashing power, thus being able to directly manipulate the transactions pushed to the blockchain. If, at any moment, a node gets that hashing power - which is very unlikely to happen, with the current difficulty - Bitcoin, as a store of value, will be compromised. Whoever has 51% of the hashing power will be able to double spend, create new tokens out of thin air, etc.
Like I said, with the current difficulty, this situation is very unlikely.
Yet, in my opinion, Bitcoin is getting slowly hijacked by a collusive group of big actors, and this attempt is fueled by the greed of (mostly unaware) candid investors.
Bitcoin, Bitcoin Cash and Bitcoin Gold
When the prospect of Bitcoin Cash appeared, it was "sold" as a technical improvement over the current technology. Alas, not much was changed in the codebase, other than just forking the legacy blockchain. In almost any way, Bitcoin Cash and the Bitcoin blockchain are identical.
Truth is, many altcoins are just colored coins on top of a Bitcoin codebase. Some of them have significant advancements, like Slimcoin, for instance, which is built on top of Bitcoin, but which introduces a completely new minting algorithm, Proof of Burn. But many of them are just identical clones, with the logo and name changed, or some minor parameters, like block generation time and so on.
What differentiates all these altcoins from Bitcoin is trust. Bitcoin is trusted to be transparent, decentralized and independent. When any of these 3 qualities will disappear, Bitcoin price will drop to the level of the lowest altcoin.
Alas, I think we're in the middle of an attempt to get rid of the one of these three qualities. Namely: decentralization.
How Can Bitcoin NOT Be Decentralized?
Suppose you are a mining company and you created some strong bonds with a company producing mining hardware. Also, around the house you grew in time a nice little exchange.
Things are going well, you are making good money, but you want more. Getting 51% of the hashing power is out of the question, it will be too obvious - and it's impossible anyway (at least for now).
So what you do?
You invent a new coin - a colored coin, basically - with a name containing Bitcoin in it (like Bitcoin Cash, or Bitcoin Gold) and then you prepare to fork the main chain into the one producing this coin.
And here's the core of the plot.
All the exchanges (which are probably incentivized by the mining farms and mining hardware companies) agree to do something absolutely strange: they will give away to each Bitcoin holder, the equivalent amount in the new coin.
Just like that. Out of thin air. There's no actual exchange going on. Just new number written in an account.
This tiny little thing has two fabulous consequences:
- demand for Bitcoin will increase (because everybody wants to get more Bitcoin Shitcoin), hence Bitcoin price will increase (aren't we seeing this already?)
- the price of the new coin will be determined, in a certain way, by this new high price of Bitcoin and by the promise that it will be better than it, hence, it will start not at $1, but probably at $300.
What happened in just a few microseconds is that a new coin appeared and this new coin, apparently, has a market cap comparable to Bitcoin's.
Do you see it now? Doesn't this sounds worryingly familiar with central banks printing money out of thin air under the promise of a better economy?
As of now, these Bitcoin Cash and Bitcoin Gold don't have any hidden parts, but there's nothing staying in the way of them becoming mineable only with a certain mining equipment. Or just in a certain mining pool.
And just like that, you don't have a decentralized Bitcoin anymore. You have a new centralized currency, controlled not by the banks and governments, but by Bitcoin whales, mining farms, exchanges and mining hardware companies.
Please keep in mind there wasn't any 51% attack going on. Just good ol' human greed.
I may be just rambling here, but I think this potential scenario deserves at least some thoughts.
I'm a serial entrepreneur, blogger and ultrarunner. You can find me mainly on my blog at Dragos Roua where I write about productivity, business, relationships and running. Here on Steemit you may stay updated by following me 
You can also vote for me as witness here:
https://steemit.com/~witnesses
If you're new to Steemit, you may find these articles relevant (that's also part of my witness activity to support new members of the platform):