Do we need another sidechain? Liquid and Rootstock already exist, while Drivechain still in development.
I think the crucial difference is the way how the sidechain would be to be implemented, according to the information available until now in the
whitepaper.
SatoshiVM is thought to be a rollup, this means it's not a completely independent sidechain but rather a method to bundle transactions so they occupy less space on L1 (but regularly injecting data on L1).
While you're correct, most people only care whether how well it works and it's adaption (e.g. which wallet or exchange support it). And with no detail how "sequencer" (party who create block) is chosen, i expect many enthusiast won't bother trying SatoshiVM either.
RSK and Liquid are currently federated, but afaik their goal is not to use rollup technology but eventually become completely independent sidechains with 2-way pegs, so apart from the deposit/withdrawal transactions, no information has to be stored on L1. This is of course a much more ambitious goal.
At least for RSK, Bitcoin TX also created on these occasion,
1. Merged mining,
https://dev.rootstock.io/rsk/architecture/mining/implementation-guide/.
2. Powpeg/federator members update,
https://dev.rootstock.io/rsk/architecture/security/.
Although obviously it has no noticeable impact on TX fee or blockchain size.