Besides, there are a few L2s with smart contract capabilities. We have Rootstock for Bitcoin (now RSK), and Stacks (STX).
The problem is that both Rootstock and Stacks currently use the model of a static federation of "trusted" users, just like Liquid.
A BitcoinOS-based Cardano L2 with a dynamic federation where everybody could participate would be a step in the correct direction imo -- in the direction of decentralization. Of course Cardano has its own centralization risks, so it would not be a perfect L2, but at least a better one than the existing ones.
If implemented, Cardano would obtain all of the security benefits of the main Bitcoin blockchain.
That's actually not really correct if I understand the model which is discussed correctly. Cardano would benefit due to the value transferred to the "Bitcoin L2". But its security would continue to depend on Proof-of-Stake consensus.
Imagine if other coins migrate to L2s as well. Bitcoin would become an "All-in-one" cryptocurrency. Who's going to need altcoins when you can do anything on Bitcoin?
That's what many sidechain supporters think, see for example Paul Sztorc's
rationale of sidechains on drivechain.info. However, I think they miss a crucial point: Altcoins benefit from investors willing to increase their risk for a superior return than BTC. And the potential for ROI if investing in a new altcoin starting from zero (or near zero, e.g. ICOs or "buying up a near-dead coin") to achieve a significant ecosystem is still high.
(Disclaimer: I have never invested in Cardano and do not plan to do so. I dislike the "cult-like" leadership of Hoskinson and friends.)