Why did Satoshi pick 21 million?

I understand that the 21 million BTC limit comes from the block reward halving formula, but I wonder why did Satoshi pick 50 BTC as the initial reward? Why not 100 or some other number? Was it just an arbitrary choice, or was there an economic reasoning behind it, like estimating Bitcoin’s eventual value and ensuring scarcity?

On a related note, I’ve been thinking about Bitcoin’s dust limit (currently 546 sats). Could it be lowered in the future to support smaller transactions (50 to 100 sats) especially if Bitcoin’s price keeps rising post next halving? Or are there reasons why it’s unlikely to change?