Honestly, a carrier charge for this feature doesn't sound any less reasonable than a variety of other near-zero marginal cost services providers have offered in the past, but $5 seems steep since you can add an entirely separate user to an existing family plan for not much more than $10.
SMS syncing is true. But if you have a phone and you're out and about you also need mobile data, so would otherwise need a second mobile plan. Or to swap the SIM over, which was easier with a physical SIM, more difficult with eSIMs.
Phone call forwarding works when all the devices are nearby, and they can handoff the audio streams between themselves; but you can't pick up a phone call on your Mac if your phone is on the other side of the world/at the office/etc; the feature we're talking about here would not have this limitation.
Ah, so the functionality that switching the physical SIM card gave you instantly and at no cost at all.
Thanks, I hate it.