Also "Soft forks are preferred because they are backwards compatible. In this case, the backwards compatibility is that if you run non-upgraded software, you can continue as you were and have no ill effect. You just won't be able to take advantage of the new functionality provided by segwit."
That is not quite true. After a soft fork, old clients may issue transactions that are invalid by the new rules, and not understand why they are never confirmed. A soft fork can also introduce new ways of storing transactions in the blockchain, implicitly or explicitly, that are invisible to old clients, as in
this example. In this case, the old clients will not see coins that new clients send them.
90% could be against segwit, yet they are the ones excluded from a fully verified blockchain.
Yes.
More precisely, if 51% of the miners decide to do a soft fork, the soft fork happens -- even if no one was told about it in advance -- and all other miners and clients have to accept it.
"if this were done as a hard fork, then everyone would be required to upgrade in order to deploy segwit and then that would essentially force everyone to use segwit."
Not exactly.
With a hard fork, all users would have to be warned in advance, with explanation of what the change is and why it is a good idea.
I there is not enough support from the miners, the hard fork does not happen and nothing changes.
If there is enough support from the miners to execute the hard fork, the users would have to be warned again to upgrade to a version that is at most K releases old by date D. Hopefully, most everybody will convert in time, and then the few lagards will be unable to use their coins until they upgrade too.
However, if a substantial minority *of the miners* remains absolutely opposed to the changes, the coin will split into new-rule and old-rule coins. Each user will see his own coins replicated in both branches, and will be able to use both independently. Is freedom of choice such a bad thing?