Going to also put out there 2 things.

1) We tend to be paranoid, so even if someone or some group does come up with another client, it's tough to get people to move because who wants to run software that controls money that has not been as battle tested as core.

And

2) If I put together a team and came out with a different client and people used it and it was popular.....Then what, is there a business plan or are people donating their time? If it's worked on as a best effort thing how does the project keep up with core. And so on.

Yes, it's possible, but it's probably never going to happen.

And, to answer the question why is it not an issue. The answer is simple, because nothing has gone horribly wrong yet so we keep using it.

-Dave