Why is One-Step-Checkout marked Blocker if Hyvä has its own checkout?

Exactly because Hyvä has its own checkout. Mageplaza, Amasty and IWD One-Step-Checkout extensions all rewrite Magento’s entire checkout in Knockout — which is the layer Hyvä replaces. Two checkouts can’t cohabit. Hyvä Checkout already does what those extensions do (one-page render, < 1 second, every major payment gateway, Klarna/Adyen/Stripe-element bridges) and ships free with Hyvä 1.3+. Removing the OSC extension also drops the licence fee — net win on cost, performance, and stability.