Schnorr signature is simpler than ECDSA, and it enjoys the linearity property, which brings more efficiency to Bitcoin. Will ECDSA be less and less popular when the Schnorr signature can be used in Bitcoin? Or more extremely, will ECDSA be replaced by Schnorr signature? Is there any advantage of ECDSA over the Schnorr signature?
Will ECDSA be less and less popular when the Schnorr signature can be used in Bitcoin?
You need to use SegWit v1 (Taproot) addresses to be able to use Schnorr signatures on Bitcoin. A desire to use Schnorr signatures will drive that adoption to some extent but there are other considerations like desire to use Taproot Merkle trees of scripts and whether your wallet supports sending and receiving to SegWit v1 addresses. One would expect usage of SegWit v1 addresses to increase over time as more wallets support it and hence usage of Schnorr signatures to increase.
Is there any advantage of ECDSA over the Schnorr signature?
This is partially answered here. As far as I know the Schnorr signature scheme is strictly superior to the ECDSA signature scheme but as the linked answer states there are reasons to still use ECDSA today like standardization and more widespread library support for ECDSA.