Would anyone like to write a GUI program(eg. web browser) using assemble language, which could provide full control over hardware?

Would anyone like to read through tens of thousand lines of the source code of a word processor to understand how it works before starting writing a word of an essay?

Sounds like fun. But what's your point?

If Linux is meant to be a command-line OS, why are there so many GUI programs running on it today?

Does anyone think that the majority of people on this planet are able to( or would like to)use a command-line based computer system ?

Linux isn't "meant to be a command-line OS". Yes, it has a command line. Yes, some distros (Slackware included) boot to a command line after you first install them. However, Linux itself is "meant to be" whatever you want it to be. Saying that Linux is "command-line based" is somewhat more accurate.

I appreciate the fact that I am able to use a graphical interface (more efficient for many tasks), while still being able to pull up a command line whenever I need to. Sure, most casual users don't want or need to use the command line to accomplish many things, but this ties into my original point:

Linux can be whatever you want it to be. You could run the whole thing using GUI tools and (almost) never touch a command line, or you can use the command line for everything. Most people don't go to either extreme, but the option is there.

Surely not.
See, the beauty of this OS is the fact that it's oriented both to end-user and network administrators. In other words, you can use OpenOffice to write a document or set up a network cluster with the same OS if you are a network administrator...
Could this be more open-minded, as an OS mentality?

Not while one can say, "They should make Linux work better for me."
Democracy in action.