but many side chains bridge to many mainnets
That's true. But from a transaction-centric point of view, a blockchain can contain mechanisms
Myself I'm not particularly fond of the "layer" buzzword. But it seems to be the most popular at this moment, more so than "sidechain".
And for me it's also different than a simple "bridge". Current "bridges" like wBTC are not tied strong enough to Bitcoin's consensus mechanisms as they often imply trust in some bridge custodian. In other words: if "wBTC" is a bridge, then I'd like another word for a sidechain or another mechanism where no direct trust is required, be it "layer" or whatever. L-BTC if we follow this definition is a bridge though, not a layer, due to the necessary trust in a Federation.
For a "layer" I would personally expect such a strong tie, like provided by mechanisms like Drivechain. I would tolerate however that the layer is based on a set of additional, but decentralized incentives. This means: a decentralized system external to the Bitcoin main chain which rewards custodians to play by the rules and punishes them if they don't, could be enough to classify it as a "layer".
i know you are trying to interpret the contracts(federations) within the sidechain which are pegged/linked to a particular mainnet justify the "layer" buzzword. but thats then assuming that a sidechain that operates with many networks is not the layer due to bridging to many networks, but within the sidechain its the contract(federation) that is the (?)layer(?) of a mainnet, thus making it appear that the mainnet must be then within the sidechain if the (?)"layer"(?) is within the sidechain.. it starts to not make sense in that way.
If the "layer" is the contract dealing with the Bitcoin-pegged token (multisig federation etc.), the rest of the chain functionality not necessarily has to be part of the "layer". In my transaction-graph centric "layer definition" the chains are independent from each other. So a chain can without problems contain a "layer" to infinite mainnets. The chains are only the technical infrastructure.
I can find only few of them useful but I would never keep any large amounts of coins long term in any of bitcoin sidechains.
From the current point of view I agree - with current transaction fees I would stay on mainchain, with Lightning as an option for smaller payments in hign-fee times.
Of course the point of this thread however is to find projects which have the potential to change that. For now, still Nomic, Stacks, perhaps BEVM and tBTC are the most interesting ones.
Drivechain is cool but I don't know if it will happen in the next 10 years on BTC. Perhaps if it's first tried on a "real" altcoin, not only on testnets.