Even without opening the back, snipers can already completely deny main. This is why teams almost always rotate through top lobby before pushing into main (unless time is of the essence). The point of opening the back would be to assist the attacking team during their actual push/uber into last when having a defensive sniper on floor/behind the point would be nothing but detrimental to the defensive team (i.e. a free pick).
Regarding the point about the nuances of opening up the top of the tube, I don't know about everyone else but onething how the last point is currently set out on logjam is what I had in mind:
But instead of the walk way it's just the circular beam we have on badlands now. I would have to test it out but the height seems pretty much perfect to keep the lamp area concealed from spammers from main. Also, even if demos can spam rollers and stickies over the top of this opening, this doesn't change anything because demos could just as well spam rollers and stickies directly on to the point like they can now (which would be a 1000 times more efficient anyways).
Regarding your nostalgia argument.. I guess that's always going to be a problem to be honest. Some people are always going to be upset that the map is being changed from its original inception but I feel that's just a vocal minority. If you're really worried about there being a negative backlash at these changes then I would suggest just opening the back first and gauging the community's reaction to the change. From what you've said, it sounds like it would require less work for yourself out of the two changes (and it alone should still make getting cap time less of a commitment for attackers)