You misunderstood. (And, actually, some computers do what you describe, both GNOME and KDE support this; kiosk mode; often used on thin clients.) Various applications always run on the computer. Some might run in the UI. I'm not arguing only one application is allowed at a time. I'm arguing a user, in general, wishes to focus on one application instead of seeing multiple. IOW, a kiosk mode which does allow (several) background tasks [multitasking] and does allow non-full screen [desktop environment] but not by default. It is assumed the user has full focus on the application. This way, the application can take more advantage of touch screen, finger-based and there is more space more e.g. virtual keyboard besides the actual application.
Speaking of the virtual keyboard. Have you tried the Python + QEdje virtual keyboard? The focused key pops up, and its not selected until the user leaves the finger off the touchscreen. This is also a good, usability related feature. The designer can assume the user approaches the key the user wishes to tap, but not necessarily accurately. Hence, this is a good solution for this problem. Here is another demo, showing QEdje + QZion (by INdT developer). This shows even more the powerful possibilities of QEdje. And don't forget... Edje is fast!