Let's say I have 10 BTC.
Then I exchange it all for cash.
What happens to the 10 BTC?
Does the 10 BTC "keys" or hashes or whatever they are called
get returned to the system and "regenerated" as new bit coins?

Or is there now only 21 million minus my 10 BTC ?

Yup...it"s the people selling the mining hardware that are making mega beaucoup bucks.
Same story...the shop keepers that sold picks and shovels to the gold rush miners at
high prices are the ones that got rich.

Back in the California gold rush days, it wasn't the miners that we're getting rich.
Sure, there was an occasional miner that got lucky and struck it rich.
But the majority of miners struck out. It was the people that sold goods and
services to the the miners that got rich. The bars...the Ladies Of The Night, the
Shop merchants....the dry goods stores....and I suspect that it's much the same
now and from what I've seen so far it ain't changed much.

If you wanna make good BTC then sell a good or a service for BTC instead
of mining for BTC.

None the less...I'm of to download a mining client and see what fun I can have