There are multiple aspects why Bitcoin would make a horrible national currency.
1. Transaction speeds and costs
Bitcoin transactions are slow and expensive. With around 10 minute block times and 6 needed conformations, it takes about an hour for that transaction to happen.
Transaction costs are too high. The massive computational power it takes to mine a block means it consumes a lot of energy, and lots of hardware is needed. Someone should really apply LCA (Life Cycle Analisys) to bitcoin, to show how polluting and unsustainable Bitcoin really is. Because its not only the power produced that pollutes, but the usage of the infrastructure, and the manufacturing of the hardware also should be taken into account.
There are other coins, that specialise in transaction speeds and costs, which all surpass Bitcoin in these aspects. Steem, Ripple, NEO, EOS etc. make Bitcoin look (and be) completely outdated. They are fast and cheap, making them optimal for the role of a national currency.
2. Scalability issue
Bitcoin has a scalability issue. With increasing difficulty, it will mean the costs of mining, and making transactions will increase inevitably. It would take horrible amount of energy and hardware to make the infrastructure that can handle the amount of transactions that happen with dollars. BTC mining already takes more energy than Venezuela, and it is really close to the amount of energy consumed in Chile. The mining takes around 9,74 million US Dollars every single day! How is that feasible in the long term?
(Source:
https://digiconomist.net/bitcoin-energy-consumption)
Other coins mentioned above has much better scaling, as they were designed to be scalable.
3. Limited supply
Limiting the supply of a currency is not a good idea in the long term. Without limiting the supply, we eill have inflation, which is really important, and people acutally cant even feel how important it is for the economy.
If you have a limited supply, and combine it with the increasing difficulty of mining, you will have strong deflation. And take a look at Japan to know why is it so bad.
4. It is not locked to location
Bitcoin can hardly be a national currency, as it is hard to imagine it to belong to a country - it was designed to be the very opposite: to be a global currency, not a national one. Mining operations happen all over the world (mainly China and Russia), having those miners accumulating vast amounts in Bitcoins. Does the US really want high percentage of its national currency in Russian and Chinese hands?
+Other thoughts: The value of a national currency is provided by the economy, and assets backing it, whereas Bitcoin is a highly speculative right now - its only value is given by the technology and the cost of mining behind it. It might change if it would be a national currency, but the reasons I listed would make Bitcoin a really bad contestant for that role. Other, above mentioned coins are better in nearly all, maybe every aspect, and I would see them as a better option.
If there would be any other coin that replaces the US Dollar, should be very similar to it, as of now, it is the optimal choice.