In this world, governments and big corporations might cease to exist as everything will be guided by both Blockchain technology and AI.
I don't know why big corporations would cease to exist. Bitcoin and other cryptocurrencies created a new market for big corporations.
For example, mining. You can't just make a start up company and start manufacturing a new asic or GPU, as those hardware are very complex. It is normal that two or three big corporations dominate that market.
Additionally, hardware wallets. We can already see 2 main companies manufacturing hardware wallets.
You can also see other big corporations in exchanges market.
Big corporations will not cease to exist...
The idea of a truly decentralized world is not just replacing fiat with a blockchain based currency and call it a day. The idea is, for example, within governments, that things like bills and so on are transparent thanks to blockchain solutions, things like government's bonuses and payouts CAN'T be laundered out of the way or hidden because the blockchain is public and anyone can follow where the money goes. Which would lead to less corruption.
That's point #1, and #2 would be, some big corporations THRIVE on govmnt corruption (see AT&T for example), if the scheme of payments, money or w.e would be changed to a decentralized one, I'm sure as hell AT&T would have to reform their way of doing business. Other businesses as you say like Nvidia, AMD, Samsung, etc, they cover specific markets and are COMPETITIVE at it, at least to an extent, they don't depend on monopolizing via corruption which is what the blockchain can and WILL solve.
It's not just as simple as replacing fiat, it's really a change in the way that public processes can be made accountable.