At that time I read some interview with one of the creators of BCL, I think it was with ScottGu.
IIRC, he said something similar to "For some classes, we regretted making them sealed, and for other classes we regretted that we did not seal them."
So, my assumption is that it was a constructive solution that may not be completely transparent to us BCL users.
On the other hand, perhaps about 100k + the user knows more about this.
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