I would not want online registration for panels, because there would still be people lining up for the no-show spots or those spots not included in the online aspect. Also, people would line up to get the front or center rows, so I don’t see that as really saving anyone any time. And it is kind of fun to line up early with a few thousand other Disney nerds
Also, there are multiple lines each day, one for the show floor/shopping opportunities, a line for the large hall panel, and lines for a popular first thing regular panel. While the crazy overnight line last year was for live action, all of them have a lot of interest.
With 70,000 or so people trying to get in each day, that takes time to get everyone through security and ticket scanned. I’d rather line up early to get to the show floor at openings, than to get in late because of hoards of people waiting to get in at opening.