You can keep bitcoin core perfect as is.

Bip300 just enables a simple opcode that allows for hash rate escrow so miners can execute a peg out from sidechains without any middle men involved like a federation. (can be removed later if people don’t like it).

The benefit for Devs is they can test all there crazy ideas on sidechains without affecting mainchain. Leading to more innovation, better security and brainpower working on bitcoin. There’s no politics with Bitcoin core devs. Sidechains devs will compete against each other but Bitcoin core will focus on their work on core without distractions of soft forks proponents.

The benefits to users is ability to scale selfcustody to billions, since sidechains will compete against each other for users you can expect the best user experience, fastest, most private, will likely gain the most adoption. Making onboarding adoption easier instead of recommending custodial lightning wallets like we do now.

One of advantages sidechains have over lightning for scaling is sidechain transaction fees are not dependent on L1 fees so better experience and cheaper transactions for users.

Privacy is essential for an open society. Privacy is true freedom. If bitcoin privacy like coinjoins becomes too expensive to use then private transactions will only be used by the elites who can afford instead of the plebs. Plebs deserve the best privacy technology to protect themselves from those who have ability to abuse their power. It’s unlikely btc will get better privacy on L1 has its very political change but you can get Ring Signatures, Stealth Addresses, Confidential Transcations and End-2-End encrypted transactions via zero knowledge proofs on Sidechains.

For me the privacy is the most appealing part as I can’t see any other BIP offering such proposal for users.