@theme, and the custom properties we already had

Tailwind 4 configures itself in CSS, which turns out to be the same idea as the token file we generate.

@theme {
  --color-brand: #1f6feb;
  --spacing-m: 1.5rem;
  --font-sans: Inter, sans-serif;
}

/* and ours, generated from tokens.json since 2022 */
:root {
  --wp--preset--color--brand: #1f6feb;
  --wp--preset--spacing--m: 1.5rem;
}

The two are the same mechanism with different naming conventions, which means a migration is a rename rather than a rewrite — and that is a much smaller number than the evaluation assumed. It also means the framework would consume our generated token file directly, which removes the argument that adopting it would fork the design system.