What software designs require clean slate 'revolution' implementations, rather than evolution?
I can think of: borrow checking in a PL (existing code is unsafe), microkernels (architecture is too different from monokernels), and full REPLs (can't add type redefinition later).
miniblog.
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Minix and how it has won mindshare in a way that Hurd struggled to: https://www.reddit.com/r/hurd/comments/273tij/hurd_the_minix_3_microkernel/chxycem/ (due to seL4, Redox I'm excited re microkernels)