I would see the introduction of an autosave feature for the tag panel as a problematic move.
There are so many experienced users who got used to press Ctrl-S or the save button when they work with the tag panel and who might use the current behaviour of getting the old data back if you select another track without saving as a feature.
The way "well, then make it an option" would not really help - what would be the default?
If you set "autosaving" to on, you have the same dilemma as described above.
If you set "autosaving" to off, then the new users will stumble over the behaviour that led to this thread - and MP3tag would more or less behave in the same way as now.
I doubt, that many new users will switch on the autosaving once they got used to the current behaviour.
Here is a thread about more or less the same thing where the idea is to issue at least a warning: