Because it isn't about a few components, it's the entire OS that needs to be adapted to touch. That means that not only things need to be big enough to hit, you also need to change the entire workflow. One of the things that need changing are context menus because you can't use those (unless there would be a touch force/3D touch touchscreen). In the end you'll end up with something that we already have now: iOS.
The only problem with systems like iOS at the moment is the lack of powerful tools. These devices are maturing and we can see it clearly by the difference in apps we had in 2010 and ones we have now. In 2010 I didn't like the iPad but I bought one this year because of the apps that are available now. It is a great device for reading, browsing but also for doing some sketching (in my case that would be diagrams), note taking (OneNote and the likes are able to that with the keyboard or even a pen), videoconferencing, remote control/management of IT systems, monitoring of said systems, various kinds of VPN, use bluetooth for various kinds of devices like game controllers or even health monitoring tools (ECG and such), editing of photos/video, some 3D modelling, some CAD, programming/scripting and so on. Applications that formerly where a notebook/desktop-only domain. This shows that the tablet/smartphones are not only becoming more powerful but also more usable for some general computing tasks. That means that we can now take a much smaller device with is to do the same thing as with the bigger one. The fact that we can now do many things on a server (or cluster even) also accelerated the usefulness of tablets/smartphones.
Maybe we'll see the tablet/smartphone world and the notebook/desktop world getting so close that they can be merged. Maybe it might never happen because we found out that it simply isn't possible/wise to do so. At least we won't see things change in the next 5 years or so. Whatever Microsoft is doing, it is indeed ambitious, possibly a bit too much. Sometimes it is better to grow towards something then to jump on it immediately.
