Condolences. You are quite correct that its present state remains somewhat problematic. Personally, I have done this as a higher-level character helping two of my lower-level friends with no additional people. Having only three of us (two of them not much above level 50) provided enough of a challenge for us to get a good experience (as in our feelings - we got no XP points since we were using the undersized option) from the fights and easy enough that we actually finished the duty before the timer expired.

So, as others started to suggest, your best bet sounds like making a party finder ad with a very clear description of what you want.

Also note there's a "minimum item level" setting available in your duty finder options. This reduces the equipment level of everyone there to whatever the minimum allowed for the duty is. I don't have any experience, however, on how easy or difficult that would make this with a full eight people.

As for your question of why we have this design in the first place, these two special main scenario quest dungeons were designed before we had a level sync system implemented. In fact, I'd wager the results you lament in your posts are the primary motivations for implementing the level sync system. So, now that we have it and have learned from these results, dungeons AFTER it no longer suffer from this issue. Many have called for them to be re-designed, but invariably coming up with new stuff wins out over re-designing old stuff. Take heart that at least it gets better.