I'll be honest, having used lots of devices without and a few with, I am firmly in the "neat, but so what"-camp.

And I would surmise that this is overall how other companies see this: They could compete, but there's just no reason to. Samsung doesn't sell their phones and tablets because they have S-pens, they sell them because they're the Apple of Android phones, they sell brand name for a sizable markup. That's a soft goal to compete with, and hence not a good one to go after with one specific hardware feature.

Unless there's some incentive, say a new mode of UI interaction that is entirely based on stylus interactions, I can't see other manufacturers being interested in it at all.

They still make the s-pen? With no slot it's so easy to be lost

The tech behind the s-pen is made by Wacom, and they're in the USI, so I don't think it's totally impossible. Pens are just pretty niche right now, partly because the android tablet market is so lousy. I think the tech has improved a bit--supposedly they're down to a 0.7mm tip now, which is in the range where handwriting on a phone starts to make sense again. So maybe we'll see more uptake of these, especially if the foldables market grows.

The use cases I really want to see for this tech are things like an advanced calculator that lets you handwrite an integral and then gives you the closed form solution if it exists, or a graph, etc. if it doesn't; and a nice pen-driven CAD program. Those would be amazing things to have in your pocket all the time, but they're a little too intricate to work well with fat fingers on a phone.

But for now I don't think the tech is really quite good enough for phones. It's good enough for my brother-in-law, who is an animator, to use it to doodle all the time, but that's kinda it. On the iPad Pro he can do a lot more with the Apple Pencil, but that has more to do with the Apple tablet software ecosystem than with the pen itself, and Google has neglected that aspect of Android. On phones the pens just seem pretty limited.