I object to pointing to 'added sugars' as the bad guys. Your body processes added sugar and natural sugar in the same way. Fruits contain vitamins, but they are also high in sugar. So the amazing changes people report might just as well be obtained by taking vitamin pills and more fibers.
I'm not saying that a better diet doesn't make you feel better, just that 'no added sugar' is just a rule of thumb. 'Natural sugar' is not in any way better (or worse) than 'added sugar'.
How am I missing what matters? The glycemic index of most fruits is higher than, for example, the glycemic index of sweetened yoghurt. So it makes little sense to eat fruits instead of sweetened yoghurt (but it would be a lot better then, say, sweet cereals).
What matters is added sugar anything is higher in glycemic index compared to the same thing with no added sugar.
You're correct in that adding fibre would alleviate some of it but might as well eat the real thing without added sugar and take a piece of fruit. It's bound to be easier and better absorbed (containing various cofactors) compared to trying to emulate the cocktail with added vitamins and fibre.
In addition dairy is notoriously insulinogenic so not necessarily a good comparison (although non added sugar full fat yoghurt is probably okay).
I'm not saying that a better diet doesn't make you feel better, just that 'no added sugar' is just a rule of thumb. 'Natural sugar' is not in any way better (or worse) than 'added sugar'.