You can't 51% attack with money, you'll need hardware. There is no $6-$20 billion worth of hardware for sale!
You can't also pay for it with market cap, they'll need to first own that much ether then dump it to get the money which would crash the price of this shitcoin and won't give them nearly as much as tens of millions let alone billions!
It's WAYYYYYYY more then that.
You also need
1) Places to host said miners.
2) Power to run said miners.
3) Staff to configure & take care of said miners
4) While buying all the available hardware for months and months you need
even more money since due to lack a availability prices will go up.
5) Luck, because even someone with a $200 piece of hardware can ruin your plans.
6) Spare hardware because lets face it a lot of miners are not the most reliable things out there.
7) Even more hardware since as you are obtaining this hardware and setting it up difficulty keeps going up.
And I'm sure there are even more things I and others have not thought of mentioned.
There are many easier / quicker / cheaper ways to disrupt
BTC mining. They have been discussed on and off for years and yet since nobody has ever done it, it shows that it's not really viable to do.
-Dave