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What threat model assumes it is easier to attack fTPM than the OS that is running?

Once you are online Bitlocker does nothing and you can just attack the OS and bypass all of its protections trivially...

Your threat model here assumes arbitrary code execution as a starting point of an exploit, the security game is up for the PC once that occurs.

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Any thread model should take firmware into account. There have been examples of unauthenticated remote code execution against Intel ME[0]. This isn't theoretical, this isn't an assumption, this has happened. Those exploits are independent of the OS running, with certain configurations the system doesn't even need to be switched on, just plugged in. With fTPM as an ME module (not all fTPMs are such) this provides an additional huge attack surface.

[0] https://mjg59.dreamwidth.org/48429.html

That vulnerability allowed unauthenticated access to AMT, not arbitrary code execution on the ME. I agree that an ME-based fTPM has a larger attack surface than a dTPM, but we haven't seen many cases where that's had real-world poor outcomes.

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