If mining gets unprofitable will it expose the system to 51% attacks?

[Kolder]

I guess it could work, but the question is why would anyone do that? If you had the power to do that, you would almost certainly own a ridiculously large amount of Bitcoin. Firstly, you would spend a lot of money, and you would essentially devalue and destroy your own currency, leading to millions of dollars lost. I can't see that ever happening.

It will definitely work but i think only bill gates or people listed on forbes top billionaire can do that thing, Because no ordinary people will allow to destroy a currency where they invested, It requires a billion $ funds to able to destroy bitcoin. But i guess that billionaires will not gonna risk on investing in bitcoin since it is not a very stable investment for profit.

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[VentMine]

There are currently 5 major mining pools that are somehow miner manufacturers themselves and they thirst for each other's blood, they're constantly upgrading their hash power and will never let others to take over, even if they do it's in their best interest to stay honest and keep the wheels rolling.
And btw mining will never become unprofitable unless the whole system collapses and in an open source project essentially there are no collapses.

It's like the time line theory where the straight line goes for ever and you can't cross it or break it only you can fork and create a parallel time line in our case another version.

I'll correct you in that mining will never become unprofitable for the miners with the lowest energy cost. The system doesn't have to collapse for this to happen.

[veleten]

mining will never get unprofitable,there is this thingy called difficulty
it adjusts every two weeks on average,as soon as the mining gets unprofitable or marginally profitable
the hashrate drops,blocks are being found less often and the difficulty adjusts so that the blocks finding time is 10 minutes on average
thus leading to lower difficulty and more profit for your PoW stake,hence more people will start mining and adjust the difficulty again
it is a very adaptable system and it has proven to be working
besides even after all the halvings are done and there is no substantial block reward,miners will be profiting from the transaction fees

[FastEddie1]

Yes it would. With no profit means less players. Less players means easier to attack.

[Qartada]

mining will never get unprofitable,there is this thingy called difficulty
it adjusts every two weeks on average,as soon as the mining gets unprofitable or marginally profitable
the hashrate drops,blocks are being found less often and the difficulty adjusts so that the blocks finding time is 10 minutes on average
thus leading to lower difficulty and more profit for your PoW stake,hence more people will start mining and adjust the difficulty again
it is a very adaptable system and it has proven to be working
besides even after all the halvings are done and there is no substantial block reward,miners will be profiting from the transaction fees

Difficulty results in mining becoming the least profitable it could possibly be while still working.  That means that it'll only be profitable for people who are established and have built up big businesses.

Basically, it'll still be profitable but that doesn't mean it won't leave the system more vulnerable to 51% attacks.  That's why Jihan Wu controls such a large amount of the hashrate (because mining becomes less profitable -> less people mine -> system is vulnerable).