If you have any reserve currency, you can convert it to something you need because many other countries accept it for trade. That’s the whole point of being a reserve country.

The concern with dollars now is that if US has a beef with your country then they can just steal your reserve funds and cut you out of trade. This obviously isn't a concern for the vassals, but it's increasingly becoming a concern for countries that want to retain their sovereignty.

The yuan will only “win” if they can prove that their currency is more stable than other currencies, and that will take decades to establish trust.

We're already seeing lots of trade happening in yuan, and the trend is only growing. It's a self reinforcing process because as more countries use yuan for trade the more it becomes legitimized and thus more attractive for other countries.

The Indian rupee isn’t a reserve currency, the yuan is.

Yuan is not a reserve currency at the moment.

That’s a bit hyperbolic.

It's absolutely not, economic fundamentals are terrible in both countries, and standard of living is rapidly declining. UK issues go far beyond brexit which has become a popular reason to blame all the problems in UK on. Brexit simply accelerated many underlying trends that have been developing since at least Thatcher days.

While the problems may be reversible in theory, it's pretty clear that such reversals are not really possible given the political climate in these countries. The trends that are happening will continue accelerating going forward.