That is not necessarily the case. There have been several 51% attacks against certain altcoins that resulted in financial gains by the attackers. Furthermore, these attacks did not result in the death of the coins.
Some of the altcoins are bitcoin gold in 2018 and 2020, satoshi vision (BSV) in 2021 and also Etherum classic in 2020.
One of the factors of this attack against this alts coins and why its is almost impossible to occur to bitcoin can actually be the type of miner used, the GPU miner is more vulnerable than the ASIC secured mining networks. Also POS is said to be less vulnerable than POW