Bitcoin will not be $20000 in 2019. It is possible to get over $10000, but not that huge chances. $20000 is almost totally impossible.
The next station that Bitcoin has to surpass is $6000, that might be reached soon, but the point at $10000 is very difficult in next few months.
For now, even the point at $10000 is nearly impossible. I don't think bitcoin can hit $10000 anytime in the current year.
This is a huge exaggeration to say that anyone could pull off a successful 51% attack on the Bitcoin network. You need the hardware and all the other resources. who has that and once somebody has it, why not just use it to mine it?
Bitman can do that if they want, but I don't see such attacks bring benefits to Bitman, or maybe others. Such attacks, if happen will only bring negative things on bitcoin and crypto, it is bad in general and everyone will be affected.