If segwit reaches locked-in, you still don’t need to upgrade, but upgrading is strongly recommended. The segwit soft fork does not require you to produce segwit-style blocks, so you may continue producing non-segwit blocks indefinitely. However, once segwit activates, it will be possible for other miners to produce blocks that you consider to be valid but which every segwit-enforcing node rejects; if you build any of your blocks upon those invalid blocks, your blocks will be considered invalid too.
This is the general behaviour of a soft fork: if a majority of miners adopts a soft fork, as a minority miner, you have no choice but to follow, or become insignificant.
Remember the definition of a soft fork: a soft fork is a protocol change, such that all what happens under the new protocol seems still valid under the old protocol, but on the other hand, what used to be valid under the old protocol isn't necessarily valid under the new one.
For instance, a typical soft fork it to black list addresses or to turn back former transactions (what is supposed not to be done, but it can, with a soft fork). The old protocol allows these addresses to transact ; the new protocol doesn't. Any new block that contains these forbidden transactions, will be considered valid by the old protocol, but invalid by the new one. As such, if you are an old-protocol miner, and you make such blocks, it will be orphaned by all new protocol miners. If they have the majority hash power, it will ALWAYS end up being orphaned. On the other hand, old protocol miners will build upon new protocol blocks without problems. They will not orphan new protocol blocks. This makes that old protocol miners will always end up losing in majority acceptance of a soft fork. A soft fork accepted by a majority IMPOSES ITSELF upon the rest.
This is totally different with a hard fork. With a hard fork, new protocol blocks are considered not valid by the old protocol. As such, if a fraction of the miners applies it, it will make a new chain, on which old protocol miners will never build. The old protocol miners will continue building the old protocol block chain and will not suffer from the forked chain that the new protocol chain miners are now building. Even 10%-90% or 90%-10% splits, nobody is FORCED to follow another protocol than what he wishes. The chain that is being mined is always mined with full consensus, but the price to pay is that there are now two chains (which is normal, there are two non-agreeing consensus groups). With a hard fork, nothing is imposed upon nobody. With a soft fork, the majority imposes its will on the minority.
