Generally speaking the hash rate will drop very hard because of negative ROI of mining (let's say $60?), at that point the bitcoin blockchain network might stop processing any transaction.
Then no one can send or receive any bitcoins anymore.
That makes no sense to me at all. I was here when BTC was worth pennies and it worked fine. If mining becomes unprofitable then miners shut down some of their mines. This is how actual mining works also. It is also how this system was designed. You may want to read "the white paper" by Satoshi Nakamoto. It may clear up some of your confusion about how bitcoin works.
the only difference i could see with the past, is a risk of 51% attack that can become more prominent, because now there are big farm that can control easily the network if the diff fall too much
Now that is a reasoned criticism. I was surprised myself to see how quickly mining went to an industrial scale. It has really run ahead of adoption. I do think that there are powerful factors that work against a 51% attack however. The most important being self interest. A mega-mine that somehow gets a majority of the hash power could launch such an attack. But what would they gain? They would get caught, it would not go on for long and would, IMO, greatly hurt the price. In the end it is hard to see how they could profit from it. Especially having invested so much in infrastructure.