Yet the percentage of legacy users and services still stubbornly holding on.
I think they will quickly reconsider when the next bull run happens, the network gets congested again, and the fees soar. Many people will adopt new things only when they absolutely have to, at the last moment, when they feel the pain and see the solution that eases it.
but the fact that they can't transact/spend to native SW unless using the same client already must mean some users are shorn off
You can provide, alternatively, a Segwit P2SH address to those legacy clients, the net result will be just the same. With a text underneath a bech32 address: "If your software doesn't understand this, use this address instead: 3........ Oh, and by the way, consider upgrading your software to the latest version."
Or you can just provide only the P2SH address. Even if you use P2SH addresses alone, you still use SegWit.