You do have a point there, mate. The bigger a PoW blockchain is, the harder it will be for the attacker to successfully perform a 51% attack and alter transaction history. Considering how Bitcoin is the world's largest cryptocurrency in hashrate, it's extremely expensive to achieve such an attack. However, the real problem lies when a PoW cryptocurrency becomes smaller in hashrate. If at any point in time, Bitcoin becomes smaller in hashrate, then a 51% attack would become cheaper to perform, violating the principles of immutability.
As such, a cryptocurrency or blockchain is immutable if there's proper hashrate distribution among miners. Considering that it’s possible to revert transactions and perform double-spending attacks, immutability within Bitcoin or any other cryptocurrency is not guaranteed. Which is why, I believe that immutability is a myth as transactions can be easily reverted (like ETH's DAO hack) when there's a high concentration of hashrate within a single entity.
Nonetheless, I hope that Bitcoin remains immutable forever. Otherwise, people would lose trust in it as a decentralized cryptocurrency for daily payments without middleman.