So scarcity is a great factor, but 21M Bitcoins is not even the true number, we will probably never have more than 17M active coins at a given time for >8 billion people! Imagine what owning 1 single BTC will mean in the next years, owning a single Bitcoin will mean a lot, it's gonna be huge. There are >51M people who are millionaires right now, they won't even be able to own 1 BTC each. So having values like $500K per BTC will seem pretty normal, people will dream for $40K per BTC in the future.
Yes, that's true. Having a bitcoin may be sound as a great wealth in the next decade(s). As for the number 21,000,000, as said, it was never exactly that many. Specifically,
excluding the genesis block that didn't create a spendable output, after the 34th halving, there will be exactly 20,999,999.9769 in circulation, where millions of them will have been lost.
The supply does not decrease. It increases approximately every 10 minutes until it nearly reaches 21 million. The supply only decreases when the amount lost exceeds the production, which is unlikely to happen until the subsidy goes to 0.
A study shows that it does, actually, decrease. Specifically
Timothy Peterson claims that 1,500 bitcoins are lost each day, meaning that on average the supply decreases with around 400 bitcoins every day. Since the genesis block, around 4% of the available supply of bitcoins has been lost each year. Lots of years later, when the block rewards some thousands of satoshis, this loss may be seen more clearly.