Soft forking is dangerous in general because it does not require the consent and acknowledgement of the entire network. Dramatic changes can be imposed by just a handful of devs and a handful of mining pool operators. The changes they soft fork in today may be OK, but if you establish it as a precedent... there's no telling what could be soft forked in without node operator's consent, or indeed even without them knowing anything had changed at all.

Say segwit is activated by the handful of pool operators, 1500 nodes upgrade to segwit capable software... you're left with 4500 nodes that don't understand and don't validate the signatures of segwit transactions. This bifurcation of capabilities of the node network is something entirely new to Bitcoin.

A hard fork is the only moral way to make serious changes to the Bitcoin protocol. If Core, bitcoin.org, reddit, this forum and everyone else was aware of what was coming, the hard fork would be no problem, altcoins do them all the time, and Bitcoin has even done one (accidentally) on the move from 0.7 to 0.8.