Yah, hmmm.... Not all troops. I can point to some that don't seem to have any difficulty.
Da necessary ingredient seems to be an active PLC and genuine responsibility and leadership for the older scouts. Often, but not always, accompanied by an older scouts "high adventure" type program of some sort. The real key seems to be real responsibility. IMO mixed-age patrols have a slight advantage here, because then the PLC becomes the home of responsible older scouts, rather than a weird mix of capable boys and newbie PLs.
Your point about the "age problem" for public schools as partners is a good one, though. Kids don't often want to go back to participate in their middle school. That's somethin' worth thinkin' about carefully. Yeh could do a dumbed-down middle school Boy Scout program and then HS Venturing, but I'm not sure I'm a fan. Yeh could have da HS charter troops, and then use 'em as outreach for middle schoolers. That might make for a really robust program, eh? Sorta like how talented middle schoolers can take HS classes. Might really ease the HS transition for a lot of kids, which is a big one.
It would be great if the Catholic high school had a unit. That would really make the transition much easier. But that is more of a district discussion, since the high school has a different CO.
I think many of our parents would feel a little uncomfortable going directly from an elementary school Cub Scout unit to a high school based troop with a different CO.
There is also the question of distance. Catholic high schools are few and far between. We don't have one in every town.
I think it would be more practical for us to have a unit in both the middle school and the high school, and let individual boys transition whenever they feel most comfortable.