I'm not so sure about this, to be honest. If that were the case then I doubt we'd still see cellular iPads, especially iPad Pros.
Personally I would love this and think there's very much a point to adding LTE to the next MacBook Pros. I use iPhone tethering now, and have used it regularly since it was first introduced. However, I've never found it to be a truly great solution. It has always drained my various iPhones' batteries so fast, even my brand new iPhone 6S, and because of that issue it's just not a great option for serious on-the-go daily use. I either have to tether very sparingly, accept the fact that my iPhone (i.e. my sole data connection) will drain significantly, drain my MacBook Pro's battery faster than I otherwise would (and remember to carry a Lightning cable) in order to keep my iPhone alive, or just not use tethering at all and go without data access on my MacBook or use a potentially sketchy public access point when I'm out. Not great choices, and I hope Apple sees this. I'm sure LTE wouldn't be cheap on MacBooks but I bet there would be people who would pay for the access. I know I would. Now that their Apple SIM technology has been refined with the iPad, this could be a great time to introduce optional LTE connectivity with the Pros in a way similar to how the iPads handle it. Who knows if they will do it, but it makes sense.
In a way I almost see iPhone tethering as being similar to what Apple Watch proximity unlocking of Macs will be once Macs get TouchID - a useful feature, for sure, and something many people will still use anyway for various reasons, but also somewhat of an alternative, so that people who don't have newer hardware can still make use of these features. iPhone tethering is still great for people who use data sporadically, too, so I'm sure the feature won't go away.