Learn English – What does “there’s must be” mean

Backing up a bit: "there's such a thing as X" means that X exists. "There's no such thing as X" means that X does not exist.

So:

There's such a thing as a zebra, but there is no such thing as a unicorn.

As for "such a thing as too...", it's a way of saying that too much of a good thing is a bad thing. For example, on my fifth birthday I learned that there was such a thing as too much ice cream. Unfortunately, so did the living room carpet. :)

Now, have a look at these:

There's no such thing as too quiet.
There's such a thing as too quiet.

Both of these sentences have the implied assertion that quiet is desirable. However, the first sentence is saying that absolute silence is the speaker's ideal, and the second is saying that while a certain level of quiet is desirable, too much quiet isn't.

It's a more succinct way of expressing "you can do X if you (want/choose/decide) to do X."

...or a pinch-runner if the batting team so chooses.

Can be reworded as

...or a pinch-runner if the batting team chooses to do so.

Or

We can still watch the concert on TV if you so choose.

can be reworded as

We can still watch the concert on TV if that is your choice.