I hear what you're saying but I don't think it's as simple as that.
For example, Rosetta, let's say ESA kept using it until it was out of fuel and then somehow released all the information needed to communicate with it and control it. Amateur radio nerds* would have to organise people and (very expensive and specialised) hardware to communicate with it. They would need a
big transmitter I'm assuming, like something from the DSN.
And if they did all that, and managed to assume control, then what would they do with it? They can't point it as it's out of fuel, and pretty soon it'll be too far from the Sun to generate much power so most of its instruments won't be very useful. Even while it's close enough to the Sun to power some of it's instruments the measurements wouldn't be anything too significant as the space between Jupiter and the inner Solar System is fairly well traveled.
If all they get out of it is the challenge of communicating with it then that's great, but is it worth the hassle?
Don't get me wrong, I loved following the 'amateur' attempt to communicate and control ISEE-3 a couple of years ago, but I do think it's a good thing that we try and clean up after ourselves and not leave defunct satellites throughout our Solar System these days. I know it seems like a waste but they (NASA/ESA) don't do it just to be nasty
Obviously Cassini/Juno/Galileo/etc. are a different story, they need to be destroyed in a controlled way and that needs to happen while they still have control and fuel. That means a controlled atmospheric entry so you can't just leave them to "take decades to spiral to destruction" - doesn't work that way unfortunately.
* used affectionately
