8 Barons. Ulren in the middle. Okay, we have a megaman thing going where we tackle them in any order we want, then take on wily Ulren. Cool!

See how you're immediately imposing expectations on something based on a completely unrelated experience? Don't do that. Megaman has fewer variables to work around when designing boss encounters. Destiny has to accommodate the established game mechanics and leveling system.

Go to adventures. One Baron is level 440. I am 390.

Maybe they're set up that way due to complexity of encounter, or as a guide on suggested challenge, or any of a myriad of other reasons. 390-440 isn't a massive difference, and at 390, you should be able to tackle a 440 challenge with a Fireteam, as the game is meant to be played.

The correct way is to have all of the barons be at the same light level, and when you kill one the remaining increase by 10 or something. That way you get a steadily rising challenge, yet can do them in the order you want.

Assuming they had the same goals that you expected them to, yes, this is one of the many ways they could have designed it, and it would probably be better from a gameplay angle if they did. But they clearly didn't have the goals that you assume they did. They probably just wanted the encounters to be as linear as possible, while integrating them into the Adventure system, so you can knock them out in the game world, with some freedom to choose.

I still agree that difficulty levels in Destiny are stupidly implemented. If I want an actual challenge in any mission, I have to screw around with my power level. I really wish you could choose a relative difficulty, like +30 power level.