I don't like Zcash for several reasons, and it has even worse history than monero, but some security experts like Edward Snowden thinks it's good for privacy.
At least for Edward Snowden, it could be because he involved on Zcash creation. And when it happened, Monero still at rough start.
For now, Litecoin's MimbleWimble implementation sounds the most interesting to me, but I believe extension blocks were extremely unpopular in Bitcoin in the past, weren't they?
You're right. Here are few past discussion that i could remember,
Superspace: Scaling Bitcoin Beyond SegWitAuxiliary block: Increasing max block size with softforkWhile it's interesting approach, it's crude way to increase blocksize and add another technical complexity. IMO it'll never happen when increasing blocksize is the
only goal.