This isn't an article about fearmongering. This isn't related to the sways of the market. This isn't about some post-apocalyptic scenario. This isn't an exercise of backwardass mental gymnastic that loops around to bitcoin going to zero. This isn't about China banning Bitcoin mining for the umpteenth time already. This isn't about crazy environmentalists torching miners' homes. This isn't about GPU manufacturers bricking miners out of their hardware. This isn't about another coin usurping Bitcoin. This isn't about %51 attacks.
What is this about, then?
This is about the monumentality of a possibility that precedes your petty pendulum of fear & greed, your method of consensus, even Bitcoin. This is the dangling blackhole under cryptography itself.
The P = NP problem has been around and unsolved for decades. It states that a computer can make up an answer for a cryptographic problem (In Bitcoin's case, a hash for a transaction) in much lesser time than it takes for a computer to procedurally and methodically answer the problem. The former is easier to do, the latter much harder, owing to the fact that it takes a quantum computer and the longer that quantum computer crunches these numbers, the time it takes is going to exponentiate.
The reason that time solving the transaction exponentiates is because the method of approaching the problem is erroneous, and not appropriate to solve it in record time. If, somehow, we prove that P indeed equals NP, then malevolence takes precedence.
An attacker, at that point, doesn't need to %51 the network in order to produce a double-spent transaction. An attacker don't even need to broadcast a transaction. Provided with the proper method, retrospective solutions to transactions in the Bitcoin blockchain could be instated, and Satoshi's Genesis transactions could be double-spent with ease. Private keys no longer have to be brute-forced with rudimentary trial-and-error methods.
This is huge. The sole reason Bitcoin has been around for such a long time, and why it hasn't gone to $0 just yet, has just been laid bare to you: there is a massive time gap between coming up with a hash versus actually coming up with a solution to the transaction. Out of that time gap, trust in the network arises.
Trust that the blockchain is immutable and monolithic. Take that trust away, and suddenly you have a scenario in which Bitcoin could be upended down to $0.
Let that sink in.
But what is trust composed of?
To the layman, the only reason Bitcoin is useful to the masses is because of its monolithic quality. Trust that any hashed transaction, containing either a number of bitcoin or any signed message, won't change in any given length of time.
In technical terms, trust in the Bitcoin network means that any given transaction, since its inception, cannot be retrospectively solved for good, that there is no conceivable method out there to procedurally solve any given transaction in reasonable time, and that no amount of brute-forcing could produce the correct sequence of passwords to unlock a private key.
This "P=NP" problem doesn't just concern cryptography. Mathematicians think that a solution to this problem is going to unburden a huge struggle we've been facing for centuries.
Take, for example, Fermat's Last Theorem. It took two whole centuries to figure out a proper solution to this stubborn puzzle, but would've taken just a month after its statement to solve if P was indeed equal to NP. Indeed, if these two were equal, the impact would result in phenomenal progression in many fields, not just maths. The unfortunate byproduct of which, however, is the true death of Bitcoin.
What can we do?
Well, not much, really. Bitcoin's whitepaper is based in its entirety that the field of cryptographic hashing is going to be viable forever. To lift the network up and abandon this field is to tamper with Satoshi's scriptures.
A migration to PoS or any other method of consensus is meaningless. So long as this blackhole of a problem dangles under cryptography as a whole, doom is impending whatever it is that you do.
Conclusion
Is this it, then? Turns out, our natural barrier of %51 won't do us any good. The good news is that this P=NP problem was first posited formally in 1971, and no significant progress towards a solution was made ever since, and I hope it stays that way, at least until the legacy financial system collapses under its own greed.