It is not a worry in the BTC network. The cost of carrying out a 51% attack is unthinkable and the chances of being successful is not guaranteed, so if you sum it up it makes it impossible. Who wants to spend billions of dollars to purchase millions of Asic miners, and spend thousands of dollars on other things like electricity, coolers, building for the equipments, etc, just to attempt to gain more mining hashrate than all of the network, which may fail. If we even agree that the attacker successfully gains control, but for how long, in a secure network like BTC i don't think it would be too long before other nodes and miners discover and a solution is immediately implemented, which would leave the attacker with egg on their face after spending so much for nothing really.

In a small and shady network like BTC Gold for example, a 51% attack can and has already happened in the past, but the security of the BTC network and the cost of an attempted 51% attack on it makes it impossible and not a threat or worry.