Do you think all dormant wallet (long inactive wallets) are gone forever?
No, there are two possibilities. It might be the owner has lost the private keys to the wallet or he's not yet ready to sell the Bitcoin.
As we are not certain of Satoshi's return, should we be bothered about his dormant wallets?
Satoshi is not coming back to take ownership of Bitcoin, and I don't kown why he hasn't move his Bitcoin all this years but I'm not really bothered about it. I believe there's a reason he left it there, he could have just use a mixer and send it to a new address.
do you think lost Bitcoin can cause Bitcoin scarcity in the future?
Yes, lost Bitcoins are donations to the network and will help to increase scarcity.