bitcoin transactions in crypto are the transactions that are considered to be very good and easy to use. in terms of security it is very reliable and has very good accuracy.
until now I have not seen that bitcon will fall because altcoin I actually see altcoin still depends on the movement and development of bitcoin, not the other way around.
so there is no need to be afraid.
It's not because Bitcoin has always lead altcoins in the past that it can not be overtaken in the future. There is always a possibility that an altcoin with a unique feature (eg. smart contract, or anonymous txs) gains more adoption because of it's real life use than Bitcoin and overtakes it. Or this may even happen when a state-backed crypto is released and receives forced adoption because of the government.
Same goes for security, there is a (probably very small) chance that a bug is discovered in the bitcoin client which causes the whole network to fail. Or maybe the chinese government decides to buy tons of ASICs and 51% attack the network. These situations are not probable but they definitely are possible.
Nothing is invincible, not even Bitcoin