If by yesterday you mean decades ago, then yes, hackers could do what they wanted because software protection was minimal both in software and hardware; time and skills were what they needed most. Today every piece of software and hardware that doesn't come explicitly as FOSS is a black box that would require all the above, in addition to more expensive gear for reverse engineering or brute force attacks on encryption, therefore being a very talented techie might not be enough anymore, hence the need to ask to those in power for a change.
This open letter targets locked-down hardware such as iPhones and game consoles. Users should have the right to install anything they want on their hardware, but restrictions imposed by some manufacturers make installing software not approved by the manufacturers either impossible or extremely burdensome. Considering the difference in power and resources between manufacturers and users, regulation is the best solution to protect the users' right to choose the software they install on the hardware they own.
This seems like an extension of what is already covered by the Digital Markets Act [1].
Like allowing to remove, replace and install any software unless "essential to the functioning of the operating system or the device" (but not the entire OSs),
allowing to choose between service providers (to some extend), not privileging the first party service, and
forcing some level of interoperability (messaging, video calls).
The actual letter doesn't seem to be dated, but the earliest reference I can find to it is from the 27. April this year.
It not mentioning the DMA is quite surprising, it has much in common and was proposed late 2020 [2].
Especially considering it is directed at EU legislators.
A cynical mind might interpret this as an attempt to potentially claim credit
for something that was already in progress, but maybe there was a reason to omit mentioning it,
or perhaps the thing was poorly researched.
In any case, in good old FSF(e) fashion this aims at an unrealistic seeming ideal, but lets see where it goes.
My guess is that legislators will take one look at this, see the similarity to the DMA, and dismiss it because the letter doesn't address how that is "insufficient".
I don't think eula,Tos and appstore/marketplace rules should prevent or restrict you from publishing or using software for any device but software and device makers have every right to make it difficult by making choices like apple and objective-c+swift or people using vmprotect to prevent their software from running in VMs or be debugged/decompiled.
Some platforms dilute the hardware cost in the software or vice-versa. Sometimes the lock-in could skyrocket the platform price which could make it less attractive. I think the solution is to step back and support open platforms. But who wants to loose performance these days?
Tinkerers of today: "let's petition those in power for the right to do what we want with that stuff".