Not absurd to contend they would steal or bribe. FBI's use of sneak-and-peek black bag operations is well-documented and has been going on for a very long time. (https://en.wikipedia.org/wiki/Black_bag_operation#Use_by_the...) In the book "The Puzzle Palace", there is a section documenting how the NSA pushed to get the FBI to reinstate blag bag ops after Hoover stopped them, so that they could recover cryptographic information to aid or obviate cryptanalysis.

I leave judgement of the ethics of the practice and integrity of the organization as an exercise to the reader.

Is it actually hard? See this discussion just a day or two ago of services in China that are doing iPhone memory upgrades for cheap [1]. This involves desoldering the NAND, copying its contents to a bigger NAND, and soldering the bigger NAND into the phone.

[1] https://news.ycombinator.com/item?id=11331044

Actually doing it may be easy for those familiar with the details.

Doing it with legally-provable assurance that you won't permanently screw it up, and proving chain-of-custody issues along the way, is hard.