They could animate the unused storyboards, couldn't they?
Who is "They"? Hasbro is a toymaking company. The movie (and the cartoon) was made by other companies licensed out with permission by Hasbro. and that took a lot of work involved since there were so many distinct parties at play at the time. There was Sunbow Productions, Marvel Productions, Toei Animation, Griffin Bacal, Inc., and many more disparate entities brought together for this project, and some of which don't even exist anymore.
And it's not like all the original assets used to make the film (character models, background art, script drafts, vocal recordings, sound effects, musical score, etc.) are all readily available to work with in one place. Heck, the surviving models, backgrounds, and scripts are something fans have been slowly trying to track down for
decades, now.
To say nothing of the fact that several of the original voice actors have sadly passed on in the years since the film's release, and several of those still with us sound noticeably different than they did 30+ years ago, so any new dialogue from them (which would likely have to be recorded from scratch, anyway) for any new scenes would sound noticeably jarring compared to the vintage dialogue from way back then.
And because new dialogue would have to be recorded, that's more money that would have to be spent on this instead of just using existing voice recordings done back then that ultimately went unused in the final movie, of which there likely exists very little if any at all.
They could get modern voice actors to do the missing dialog.
But there's more to putting in new dialogue than just recording it. There has to be animation for it. And for that, new footage would have to be made from the ground up. And in this day and age, nearly all animation is now done digitally, which looks far different from the original hand-drawn animation of the 1986 movie. That clip you posted above is neat as a novelty on its own, but insert it into the actual movie and it suddenly takes you out of the movie experience due to the sudden stark shift in animation style that changes again when the movie resumes with its original footage. A whole movie like that with shifting animation that isn't trying to be an artsy film or a parody or some other deliberately outlandish experience would just be a mess.
Not to mention that the addition of new scenes would require a rescoring of the existing ones in order to accommodate the new ones. And depending on how many new scenes there would be, that could very well require a complete rescoring of the entire film!
Basically, no such extended cut of the movie has ever been made because it isn't seen as something that's logistically practical.
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