My generation was taught: "Once you have physical access, the game is over." I believe this is still true. Pretending otherwise feels like snake oil.
Disk encryption then became practical, and defeating it requires a running, vulnerable machine (for cold boot) or tampering+user interaction (for evil maid).
Secure Boot makes those even harder - you will have to compromise the TPM itself.
All of this is to say it is still possible to attack a machine with physical access, but you now have to enage in further security breaks. It's not really "game over" anymore as there are further defensive to defeat.
The same way that locking your front door or being home or parking in a guarded parking lot will be efficient in deterring opportunist thieves and even most regular thieves.
Absolute security doesn't exist but that doesn't mean security measures are futile in general.
Now I'm going to propose a more likely future scenario. Imagine you are on another planet and there is a software flaw that needs to be patched. Do you want to have to beg some billionaire millions of miles away for the privilege of modifying the code, or do you want the ability to fix it as soon as possible? If its a time-sensitive issue consider the consequences of having to fumble around with deploying signed patches, or installing a new key and updating everything to work with the new key.
None of these trust devices are perfect, they might leak data that can be used to infer keys through side channel, or can be physically inspected, etc, I think we can all agree they are not perfect. So if the security model breaks down with physical access then the only adequate solution is physical security. Putting your faith into some crypto function that will be obsoleted by time is not a winning defense. It's also possible the keys may have been obtained by a malicious adversary, rendering this mathematical security layer ineffective at best. If I can install new keys, then an attacker can also install new keys. If the imperfect security measure causes more problems than it solves, then its worth rethinking.
What problem would it ACTUALLY solve in this scenario? Maybe it could be used to mitigate a breakaway civilization by only allowing patches through remote update channels, keeping new outposts fully dependent on Earth. Potentially at the cost of real lives if that update channel is broken somehow. Though someone always figures out a way to break the lock :)
Please I don't want to hear about some hypothetical dissident of a tyrannical regime, they should have destroyed the device before capture if they had any wits. The regime would probably be forcing its citizens to run fully validated stack anyway and would be all up in their business for (red flag) installing their own key instead of government approved key... Corporate security? Lol do you think your petty secrets are worth restricting humanity by normalizing imperfect security measures? Some of these corporations need to be kept in check somehow, they have too much power already.
The only future I'm on board with is one with a robust well tested software ecosystem controlled by its users/operators/owners, not a careless conglomeration that prioritizes the well being of it's shareholders, or crushing its competition.
One such reason is simply compliance: if a device is only capable of safe operation when running within a certain spec and dangerous otherwise, regulatory frameworks might require manufacturers to ensure said specs are honoured.
While a manufacturer can and must provide the "robust and well tested software ecosystem" you yourself desire, Mrs random tinkerer might not. A device running with modified firmware might therefore lose its operating license and - best case scenario - get damaged in the process, or on account of it being dangerous if operated outside of spec endanger the life, health and/or property of its owner/user/operator.
There's no need to dream up hypothetical scenarios on distant planets when all it takes is a typo to turn an MRI machine into a waffle iron for people or an Insuline dispenser into a super effective suicide device.
Not every device is intended- or safe to be tampered with by unqualified laypeople.
It's also pretty arrogant to assume just because you know how to program to also have sufficient knowledge to be able to understand and make meaningful changes to every conceivable device in existence that uses some kind of firmware.
There's a reason certain jobs require qualification and there's also good reasons certain devices are not to be messed with by unauthorised/unqualified people either.
Maybe I've just had too many horrific experiences with software updates and that is causing some bias. Or seen too many science fiction films.
"Those who give up freedom for security deserve neither."