Assume that Bitcoin value versus the “fiat money” is of great significance and imagine suddenly discounted by 50% or more, the current pools are not in money any more, stop hashing. The current Bitcoin system will automatically set the difficulty down ( exponentially decreasing mode and easy to mine blocks using again a laptop ). Any supervisory power on earth to ensure that Bitcoin system will remain resilient if hashing conditions change rapidly?
Bitcoin is a decentralized currency and there is nobody with "supervisory power" over it.
Anyone is welcome to suggest improvements to the protocol (changing hashing rules or anything else) at any time, and can create a source code patch to any of the several pieces of Bitcoin client software to implement them. (The maintainers of Bitcoin Core or another piece of client software can do so simply by releasing a new version.) Each user can then make their own decision whether to adopt this patch or new version, or to continue using the software they already have. (Of course, they might be more likely to decide to adopt a patch if it comes from a "trustworthy" source, such as the software maintainers.)
If a general consensus emerges that the improvement is desirable, and essentially all Bitcoin users adopt the patch, then the protocol has effectively changed. So such a method could be used to make a change to the difficulty adjustment rules, should there be a consensus that such a change was necessary.
Assume the patch changes the protocol in a way that is incompatible with the existing protocol, so that the old and new protocols disagree about the validity of certain blocks or transactions. (For example, the new protocol might "lower difficulty" by saying a block of a certain low difficulty is valid in a certain situation, where the old protocol said it was invalid.) If some people adopt the patch and others do not, then there will be a problem: those two groups will disagree over whether particular transactions or blocks are valid. This situation is called a hard fork and essentially would split Bitcoin into two separate currencies: "old Bitcoin (BTCO)" and "new Bitcoin (BTCN)". Anyone who held, say, 1 bitcoin before the fork would effectively now hold 1 BTCO and 1 BTCN. The economic effects would be confusing, to say the least. Many people consider that if such a state of affairs continued for a significant length of time, and with a significant population on each side, the effect would be to destroy confidence in both the new currencies and render them both valueless.
As a result, users are likely to be cautious about adopting patches that change the protocol. They should only do so if they believe that the vast majority of other Bitcoin users will do the same. For this reason, implementing protocol changes is as much a political challenge as a technical one, and it helps a lot to have "trustworthy" people advocating for the change. So those trustworthy people may have some amount of power over the protocol, but not in a "supervisory" sense. It's less like being the king, and more like being the prime minister of an unstable coalition government.
If there was a feeling that the existing difficulty adjustment rules were not working well, the software maintainers and other well-regarded people could propose a change and hope that the user community as a whole would agree. That's what would likely have to happen in a situation like what you describe.