An upright screen that you need to use for lots of input would indeed result in the arm becoming tired quickly. But no one actually uses their touchscreen laptops like that. It’s an additional form of input. Most of the time you still use the keyboard and mouse/trackpad. But it’s very convenient sometimes to reach forward and touch the thing you want to click instead of mousing over to it (especially if you have multiple monitors). It’s also extremely convenient/comfortable for scrolling through long documents if you have a laptop on or near your lap. You rest your hand on the base/your lap/table and scroll with your thumb.

I don’t use the touchscreen on my Windows laptops a ton, but I still use it daily. And I miss it when I use my MacBook.