Oh, and by the way: Now A has 20 shares left (out of a 100 long position), C has 20 shares left (out of a 80 long position), and E has 60 shares (out of 60 long position).
Now assume that entity E is redditors/RobinHood/financial justice warriors that pledge to hold, and not let anyone borrow their shares, to squeeze the bad bad shorts B and D. So, FJW/HODLers control 60% of the shares now, and will never ever lend or sell!
A: 100 long (=20 shares, 80 lent), B: 80 short (=0 shares, 80 borrowed), A+B together net 20 long (the rest is held by C, D, E)
Well, so no A can sell, say 10 shares to B:
A: 90 long (=10 shares, 80 lent), B: 70 short (=10 shares, 80 borrowed), A+B together net 20 long (the rest is held by C, D, E)
And now, B returns those same 10 shares to A:
A: 90 long (=20 shares, 70 lent), B: 70 short (=0 shares, 70 borrowed), A+B together net 20 long (the rest is held by C, D, E)
Well, so no A can sell, again, say 10 shares to B:
A: 80 long (=10 shares, 70 lent), B: 60 short (=10 shares, 70 borrowed), A+B together net 20 long (the rest is held by C, D, E)
And now, B returns those same 10 shares to A:
A: 80 long (=20 shares, 60 lent), B: 60 short (=0 shares, 60 borrowed), A+B together net 20 long (the rest is held by C, D, E)
As you see, with only 10 shares circulating, and 60 shares in the hands of HODLers, B can happily reduce their short exposure. Let those shares circulate in this manner a bit more, and you end up with:
A: 20 long (=20 shares, 0 lent), B: 0 short (=0 shares, 0 borrowed), A+B together net 20 long (the rest is held by C, D, E)
Now, B is out of their short and flat, A has the same 20 shares it had at the beginning, and only a 20 long position now, and C, D, E keep holding their 80 shares net together.
Now, C,D can also drive down their position, we end up with A holding 20 shares, C holding 20 shares, and E holding 60 shares.
TL;DR: as long as 1 share is circulating, the shorts can reduce their position to zero, even if the majority of shares is held by never-lenders, never-sellers.
And now the market can collapse, and E is left holding the bag.