1) Bitcoin does not have a trustless sidechain. Every sidechain is trusted, even Liquid. (Read the code. Liquid has a fallback which gives BlockStream full control of funds if the pegout federation fails to agree)
2) Drivechains do not enable shitcoins. Shitcoins get eaten up by miners, by design.
The motivation for a trustless sidechain is so that Bitcoin can act as a hard immutable backbone to a broader money system. Bitcoin cannot be money, because it fails two necessary properties: scalability and fungibility.
Drivechains enable scalability and fungibility without compromising on Bitcoin's immutability, 21M supply, or security. Its a win win win.