To write interesting and realistic characters, they have to have layers, conflicting motivations and personal development and growth.

If the backstory is just more of the same, it's not really that interesting, nor very believable.

But if you can use it to show some of the facets of the character that are not as prevalent in their current behavior, maybe even conflicting with their main traits and how they are perceived today, then that's interesting, that gives them a new depth, then you want to know how and why they turned out differently and maybe even what would have happened if they didn't.

This is at the core of the writing for young Kirk and young Picard. Kirk written as a nerd and Picard written as a hothead is in contrast to their current personalities. Not necessarily diametrically opposed, they don't need to have gotten completely rid of those traits, but they are now subdued, reigned in, overcome or ripened.

So yes, Picard as seen in the present of the series IS a stable, prudent, diligent man of science and morality, while Kirk as seen in the present of the series IS a rebellious, womanizing risk taker full of hope. Both have overcome, mastered or even developed their youthful traits that provided contrast to their current personality into their current traits.