Expected network hashrate is over 100 TH/s at this time
I assume you mean 100 million TH/s, not 100 TH/s.
Regardless, who says the hashrate should be that high? The hashrate has never been 100 million TH/s. The highest it has ever been is around 61 million TH/s.
The network is susceptible to an attack greater than 51%.
Not really. The current hashrate is 35 million TH/s. The first time it was ever that high was only in June of this year. For the first 9.5 years of its life, bitcoin had a hashrate below 35 million TH/s, the majority of that time being
significantly below this level, and yet survived without a 51% attack.