That has no impact on what MS can do downstream as long as it's an open license.
If Microsoft, or Opera, or anyone else downstream wants to pull out parts of Chromium and replace them
with Rust components (including a parallel renderer), they are free to do so. It increases their maintenance cost to maintain the unique components, but so would maintaining a whole separate from-scratch project instead of being downstream.
That has no impact on what MS can do downstream as long as it's an open license.
If Microsoft, or Opera, or anyone else downstream wants to pull out parts of Chromium and replace them with Rust components (including a parallel renderer), they are free to do so. It increases their maintenance cost to maintain the unique components, but so would maintaining a whole separate from-scratch project instead of being downstream.