After segwit activation, will the hard fork revert the changes?
No. With Segwit the block size parameter changes into 'base' and 'weight'. With Segwit you get 1 MB base and 4 MB weight. If one wanted to increase on-chain capacity via this method post Segwit, these paramters would have to be changed. Traditionally it would only be one parameter (as is now).
I honestly see no problem with segwit, nor BU. The real problem is that we need a change soon. We need one now.
IMHO emergent consensus has a far greater risk than Segwit. I'd prefer Segwit or the old Classic approach (2 MB) instead of BU.