Blockstream wants soft-forked SegWit to fix the malleability problems (that would be needed for the LN, if they ever get it to work), and to force ordinary p2p bitcoin users subsidize the costs of complicated multisig transactions (ditto). But these reasons do not seem explain the urgency and energy that they are putting on the SegWit soft fork. Maybe they have other undeclared reasons? Perhaps they intend to stuff more data into the extension records, which they would not have to justify or explain since, being in the extension part, "ordinary users can ignore it anyway"?
As for SegWit being a soft fork, that is technically true; but a soft fork can do some quite radical changes, like imposing a negative interest (demurrage) tax, or raising the 21 million limit. One could also raise the block size limit that way. These tricks would all let old clients work for a while, but eventually everybody will be forced to upgrade to use coins sent by the new verson.
A hard fork based consensus mechanism, far from being dangerous, is actually the solution to centralised control over consensus.
Script versioning is essentially about changing this consensus mechanism so that any change can be made without any consensus. Giving this control to anyone, even satoshi himself, entirely undermines the whole idea of bitcoin. *Decentralised* something something.
Changes to Bitcoin’s script allow for both improved security and improved functionality. However, the design of script only allows backwards-compatible (soft-forking) changes to be implemented by replacing one of the ten extra OP_NOP opcodes with a new opcode that can conditionally fail the script, but which otherwise does nothing. This is sufficient for many changes – such as introducing a new signature method or a feature like OP_CLTV, but it is both slightly hacky (for example, OP_CLTV usually has to be accompanied by an OP_DROP) and cannot be used to enable even features as simple as joining two strings.
Segwit resolves this by including a version number for scripts, so that additional opcodes that would have required a hard-fork to be used in non-segwit transactions can instead be supported by simply increasing the script version.
It doesn't matter where you stand on the blocksize debate, which dev team you support, or any of the myriad disagreements. As Gregory Maxwell himself states:
"Anyone who /understood/ it would [shut down bitcoin], if somehow control of it were turned over to them."