You can use a laptop as a desktop, but not for as long.
Most laptops, except for the IBM and Lenovo Thinkpads, and a very few of the high end HP laptops do not last as long. Meantime between failures is significantly worse for laptops than desktops. Why? More heat in the case, primarily, and components such as LCD screen that burn out earlier.
A desktop should last 7 to 10 years with an annual cleaning, replacement of the CPU fan and CMOS battery, and perhaps a new hard drive every four or five years.
In that same time, a laptop will likely need a hard drive, a new cpu cooler, a screen inverter, and a CMOS battery every three years, and a keyboard every four years.
The expense of maintaining a laptop is three times as high for the same number of years as is a desktop.