So, I was wondering why not we have many Bitcoin Blockchains on a single Bitcoin network? Is that what sidechain means? We could have multiple Bitcoin Blockchains of different sizes... Some can be "unlimited" blockchains, meaning no limit to the amount of data that can be stored in them. While others can be limited, meaning there is limit to the amount of data that can be stored in them.
Every method of securing those sidechains comes with trade-offs.
If they are secured by Bitcoin miners -- like soft-forked or merge-mined sidechains -- they constitute block size increases. All that sidechain bloat is a big disincentive to running a full node, which defeats your purpose.
If they are secured by federated consensus or are simply altcoin chains, they are irrelevant to the question of
Bitcoin nodes. These nodes aren't part of the Bitcoin network.
If you're talking about sharding, then you're reducing "fully validating nodes" to "partially validating nodes." This greatly weakens Bitcoin's security, which is largely based on peer-to-peer redundancy.