I keep seeing this statement but can't for the life of me figure out why. You're talking about a device which has a screen that's a couple of inches across at absolute most. How on earth is this expected to replace a smartphone screen?
I think the situation over time - not in a year or two of course - will be that all the processing power, storage, communications and so on will move to the wrist, and the 'big screen' you need for certain stuff will be just a 'dumb' screen that communicates with the watch when you need it. And it will probably be a fold-out version that gives way more screen size than what a reasonably pocketable phone does today.
In other words the exact opposite of the relationship between smart watches and cell phones today where the majority of stuff is handled by the phone.
Don't forget that most websites have cell phone versions of the site available, and there is no problem surfing most of them on something like a Samsung Gear S (which actually has a higher resolution screen than the original iPhone). No problem typing on that screen either - if you insist on doing that instead of using voice to text that works very well these days. Of course you do not write long e-mails or novels on such a device, but no-one in their right mind would do that on a cell phone either.
That being said; why does a smart watch need to have a small screen? I hope some manufacturers have the guts to move away from the 'it has to look like an old-fashioned watch for some weird reason'-thinking, because that way of handling the design just leads to lots of artificial constrains that cripples a potentially great product.
If I could have something like kdarling shows in the concept renders post a little bit up - a curved large screen that wraps around your wrist - I would ditch my regular cell phone in a heartbeat.
I think the Samsung Gear S (big curved screen, sim card support, GPS +++) is the best smart watch today by a huge margin if you look at the potential and usability of the technology. But then again, it does not look like an old-fashioned watch at all and quite a few people seem to have an issue with that.