So this sounds like a good reason for game developers not to use decentralised currencies, since it makes it easier for competitors to steal players away.

It’s the same question as whether or not an office suite or a graphics editor should use an open format. There are arguments for and against. If you are open, you can attract users from other products, and being open may make users want to use your software. If you’re closed, then you get to lock users in, which can also be very lucrative.

While not participating means no "real world" money involved and an uphill battle for adoption. The game would have to be an outstanding gem, and even then markets will form.

Would it be worthwhile for a game developer to spend time on an economic system, or the game? Unless the game is an economic system...

Generally speaking, game developers need to create and maintain "fixed" economic systems to make the game fun. Games regularly suffer from ridiculous economic effects because it's too easy to get money and not enough sinks, or vice versa. Making the tokens tradeable outside the game doesn't fix this, and in fact creates even more problems - the presence of gold farmers and those who pay them can unbalance the game for others playing the game with them.

People spend their lives struggling under capitalism and use games to escape - and then you want to create a capitalist class system there too?

You can try to stop it, but that never works in the long run. Ever.

If there's going to be a social element, it will be exploited to the game's level of popularity.

That isn't to say a game can't remain closed and private with a simplistic model, just that popularity will generally weaken whatever community aspects made it great early on.