I have the feeling the Tier 1 and below do not operate from the position of fear only, but also by lack of knowledge.
I know that contract law is dispositive and there will probably be situations were new unilateral requirements will be valid. Normally the argument is that the supplier has in the past signed the “Supplier Quality Manual”, and therefor is bounded by all new CSR:s in the future.
The argument is bogus. Prior to contract acceptance every company has to fully review all requirements. I'm not a lawyer. I did learn about contracts in the 1980's working at a company involved in DoD work. Talk about complex contracts... I remember working on a contract proposal that took the company over 6 months to prepare. Complex references, requirements, etc. It was a company wide project with multiple teams.
Automotive isn't any where near as complex. And
...that the supplier has in the past signed the “Supplier Quality Manual”...
A contract is a contract and aspects such as 'intent' aren't really applicable. The contract says what it says. If a specific 'intent' was actually intended, it should have been called out/specified in the contract. I saw this back in 2016 when before a judge a lawyer started talking about the 'intent' of a contract provision. The judge said what I just said above - If there was a specific intent it should have been written in the contract. "What I meant at the time" isn't a valid argument.
I have written quite a few times in discussions in the forum of the importance of contract review. Whether a company agrees to "new unilateral requirements" being applicable to existing contracts is a choice the company has to make. I, for one, would not make that agreement if only because, unless there is a provision to allow cost/price revisions in such cases, there is no way to know how much "new unilateral requirements" would cost.
EDIT: Made some clarifications and corrected some typos.