The worst case scenario is that the cryptocurrency market can be gone forever at some point. Maybe it can happen in e.g. 100 years or so. But the same probability exists for fiat money also. Because governments are developing their own digital currencies now. And they will start using them instead of fiat money at some point. In short, it is really hard to predict whether volatile cryptocurrencies will be gone forever. Let's just be aware of everything and make our investments according to it.

This scenario is very unlikely, and if ever some countries tried to collaborate to create a blockchain, there will never be something as the same with Bitcoin. Bitcoin is the most unique cryptocurrency out there. Unlike the blockchain that the government is going to create, Bitcoin is a decentralized technology, there's no way all the government would be able to create something close like that because government would want control obviously, and that's not going to work.

bitcoin is decentralized and have nothin to do with government; people can still choose bitcoin or other crypto digital
we can took example of digital yuan but some people of china maybe still prefer bitcoin

and i dont know if the government will joining each other even united nation member itself still have non member country so its impossible

Every country wants to become a superpower and wants more voice.
They will not cooperate, they are in a competitive relationship.
Many countries have now banned Bitcoin. Bitcoin mining is prohibited.
The state banning the use of Bitcoin is also more troublesome for everyone, but now everyone is unable to resist the government and can only be forced to accept that Bitcoin cannot be used.

According to me it won’t affect our Cryptos at all.
Our cryptocurrencies are decentralised, so as like now if we hold it and sell it P2P then no one will know about it.
And if the government launches something like you have mentioned in the OP, then i am sure majority of the people will ignore it.
As people don’t like keeping their money and investments under centralised policy.
Hope this helps you Op.

They will never do it. They will never cut their own neck with their own hands. LOLZ.
Let’s think about it, why would government want their own blockchain if they have well settled traditional banking system.
They don’t need blockchain for storing the state secretes or national secretes which might get stolen with hackers so that’s not the agenda on the table.
They will prefer to run an underground operation with stone caves rather than spreading the critical information over the Internet through public ledger.
That would be last thing any government will do.

Worst case scenario I believe would be that the government bans everything blockchain. Including every crypto currency that it aids. What I know is even if create an a customized digital currency they would still need the operation of blockchain. Overthrowing blockchain would be too much a demand I believe.

that would be a beast if they be able to create like that but i dont think they can .
 as far as i know theres no one succeed in copying this two ( eth and btc ) but there can be creations that are close enough for eth , it can be because eth is less stronger than btc . your holding are too small compare to big bag holders but your worrying too much than them  .
dont worry no worst things can happen with btc and eth

Even if that becomes true, crypto is here to stay and you're not holding a shitcoin. You don't have to be worried about it at all, you're holding the best two cryptocurrencies in the market so that shouldn't be a problem for you. What you should worry about is accumulation.
While the market is starting to show again the price increases, you should keep accumulating while it's not yet at the most possible peak of it. You name the price for the most possible peak that you can ever imagine, it may happen this year, next year or so. But with what you've said, it's not the worst that I can imagine. There's more to it like shutting down of all the exchanges which is an impossible thing to happen.

Op says building blockchain? so it makes no difference that government union just make another boring crypto alternative. The most urgent thing to do instead of creating new ones is to suppress the growth in the number of altcoins. Decentralization is not a crime, scam coin is a crime. I see the founders reserve the largest amount of their coins unfairly, and they can liquidate at any time which can impoverish a lot of people.