The access points are just the start.
Like I mentioned, the ground outside the track falls away along that stretch. They have sort of cut the corner in adding some space on the inside of Turn 3. However, for the rest, you'd have to build up a massive hillside, keep it in place until the vegetation can hold it there (it's a rather rainy spot), figure out what to do with those access roads, and that's IF the track owns enough of the land on that side to do all that, not to mention, whether the NY EPA would even approve it.
Not all the ugly incidents at the Glen happen in "the Esses".
2008:
https://www.youtube.com/watch?v=6unt74Gw96s
2009:
https://www.youtube.com/watch?v=2-thoBzdwYE
So, they add more paved run-off, and guys use it for granted, which contributes to this.
2014:
https://www.youtube.com/watch?v=Jc4QX_FjeZs
(The SAFER barrier is why you guys have NO run-out at the final corner. An "antiquated track" is fine, if you drive respectfully; if you don't, not even the wide-open spaces of Riverside or Cleveland would be "enough".)
I think there are two tunnels under that stretch of the track. One leads to the interior of "the Boot"; the other leads to the outfield side of the track. The land drops away too much within "the Boot" to make messing with that tunnel anything less than a total nightmare.
As for Brainerd, the bikes had a slower Turn 9 built for them to largely mitigate the bridge issue on the old course. I think the bigger "problems" people see with the old course are the banked Turn 1 and the use of the dragstrip for the front straight. Then again, since they're easily off of Turn 10, and hit the strip in a straight line, that shouldn't be such a big concern.
(And regarding the new course at Brainerd, I just have no great desire to watch much of anything run on it.)