Should we be worried? Should we trade our BTC now for other coin like ETH?
As far the the OP goes, it's nonsense. The 21 million coin cap can't be altered without a specific change that would alter the block reward. That would require a discrete change that has nothing to do with Segwit, and such a change would face tremendous opposition from the userbase of current bitcoin owners since it would dilute the value of all existing bitcoin.
Likewise, the 51% fear is overblown. Even if the 98% figure is not a wild exaggeration (and where does such a prediction come from?), China is a huge country with many miners. (Keep in mind that the number of miners is much larger than the number of mining
pools).The notion that some of the largest will collude to execute 51% attacks apparently runs like this:
Chinese Miner A: We have hundreds of millions of dollars invested in bitcoin mining. Let's form a cartel to execute 51% attacks!
Chinese Miner B: Yes! We'll be able to double-spend whenever we want! People will be helpless as we rip off other bitcoin users with our control of 51% of the network!
Chinese Miner C: Yes! And won't it be a wonderful thing when disillusioned and panicking bitcoin users all dump their coins in response to our attacks!
Chinese Miner A: We'll revel in our domination of bitcoin even as the price of bitcoin goes into a death spiral after the first couple double-spends!
Chinese Miner C: Thus rendering worthless the hundreds of millions of dollars we've invested in mining hardware, as we kill off our profitable businesses! It's brilliant!
That, apparently, is the logic...