They can make an altcoin and voila, they'll have a blockchain.
But there's no stronger blockchain than Bitcoin's. And if that's advertised enough, organizations will come for the blockchain and will have to use bitcoin for that.
For the moment, the bitcoin blockchain is the most 'used' and it is the most secure but like I said "for the moment". However it is normal that in the future someone will build something better than the actual bitcoin (this is obvious... technology).
You mean like they built a better gold for thousands of years? Not proof, but that makes your argument pretty weak and makes bitcoin look more likely to last.
bad argument. Because gold is gold, it's just defined that way. If you improve it, it'll be something different. You can't build a better gold, it's a matter of semantics.
However, in a sense, bitcoin is a "better gold".
Your argument is like saying "you can't build a better bitcoin than bitcoin". It's a useless statement. But you can most certainly build a blockchain better than the bitcoin blockchain, just as you can build a computer better than the first commercial PCs.