I don't think it is possible for the dollar to fall, because the few international transactions that take place are mostly done through dollars. But every country has its own currency but the dollar is international so it is not so easy to lose its value.
He is talking about collapse there, not fall and I think both are not the same but to me, I think the word collapse is more intense than the word fall. If I can make an example, I can say that a collapse is like a shitcoin who dumps and never recover again but a fall is only like a dip in the price of BTC and the chance for it to recover is still or always there.
Now that we already figured out their meanings, I guess it is now easy for us to choose a proper word in order to have a response in the OP's question. Even though you already choose the word fall, it is still wrong to say that it is not possible for the dollar to fall.
There is an image about it in fact which compares if what we can buy with a one dollar now compared to before but it's not just a simple fall, rather it is caused by the inflation. If without the involvement of the inflation, there is still a small decline that a dollar can experience sometimes and just like in BTC, it is still possible for it to recover too. Anyways, the next thing that you said is the reason on why it is less likely for the dollar to collapse.
it may be that if some other better method and strategy comes then the dollar may lose its value, but this dollar has been playing a role in the transactions of every country in this world for a long time.
The dollar already loses its value but as we can see, it is still considered as the main currency of all the countries in the world, so even if other perform well, I don't think we can see them in the position of the dollar.