Didn't SegWit increase the block capacity? https://www.bitcoincash.org/ Even if used in 100% of transactions, the increase would equate to 1.7MB blocks. Thus, it is a small capacity increase at best.It will not handle exponential growth or worldwide usage. Second, the soft fork implementation results in discardable signatures, which weakens Bitcoin's security model. Third, it makes future capacity increases more difficult due to bandwidth inefficiency and quadratic hashing attacks which SegWit doesn't solve since an attacker isn't forced to use it.
For those (and other) reasons, Bitcoin Cash was necessary as a pre-SegWit fork. Segwit will not be adopted.