Or the code is specially laid out that way over the years to make this a person-years effort.

Come on, no software project is ran with that level or "what if" foresight.

I'll go with incompetence over conspiracy.

No "what-if" is needed to pull of that kind of thing. As an advertisement company, just never prioritize this separation, and do not discourage this "integration" in PRs.

You're done. Build while fresh.

P.S.: Microsoft is actively rigging its software, lastly VS-Code just for these things, but it's another can of worms I have no desire to open again.

In this writeup [1] about adding Rust to Chromium there is a note on the architecture shared state in the architure which may explain the difficulty

"... Chrome is built on foundational but really wide C++ APIs, such as the //content/public layer. We examined what it would mean for us to build Rust components against these types of APIs. At a high level what we found was that because C++ and Rust play by different rules, things can go sideways very easily.

For example, Rust guarantees temporal memory safety with static analysis that relies on two inputs: lifetimes (inferred or explicitly written) and exclusive mutability. The latter is incompatible with how the majority of Chromium’s C++ is written. We hold redundant mutable pointers throughout the system, and pointers that provide multiple paths to reach mutable pointers. ..."

[1] https://security.googleblog.com/2023/01/supporting-use-of-ru...

An expert in a field should be able to estimate with reasonable accuracy what kind of effort a certain activity requires. And to be honest "person-years" doesn't look that much at all for a feature that's so useful (but only to the user). 10 people working for a couple of months full time already qualifies.