Bitcoin is immutable because of economical incentives.
Behind every full-node and miner is a human operator making a decision or choice to carry the cost of running such an operation.
Security from decentralisation can only exist if users wish to use and validate Bitcoin, because it cannot be easily censored as centralised systems. Security (Immutability) from confirmation (PoW) can only exist, if it is paid for (by fees).
Individuals will make choices which improve their situation. The system can only work if it supports this and is reflected in the incentives for all participants.
Those who are able to 51% the network can't do it, because it'll
result in the value of the currency to drop tremendously, which causes
waste of a lot of money as there's a 100-block limit to spend coinbase
There is no clear incentives that prevent a 51% miner from double-spending or censorship. A double-spend may reduce the value of Bitcoin, but whether it is profitable for the miner (in let's say USD terms) will also depend on the double-spent amount. A 51% selfish miner who censors blocks, may benefit financially from doing so by diverting Bitcoin transactions to another domain which can be taxed by the miner.
However, user's can offer higher fees if their transactions are being censored, thus incentivising new mining hash rate to join the network and dilute the previous 51% miner.
Those who aren't miners having large capital to attack the network
can't turn on a considerable amount of miners at once as it's not a
Capital can always be reallocated to mining, but they need an incentive to do so. If users pay higher fees, this can incentivise new hash-power to come online, and dilute the former 51% miner. Users will only pay higher fee's if their transactions are not being confirmed (being censored by 51% miner).