Linux has been my main OS for about 5 years.

The amount one needs to use the terminal for for normal stuff in linux is getting less and less. The same with the amount you need to know.

For example. a few years ago if you wanted good disk performance you need to know how to enable DMA. This meant you had to read man pages and figure out if you drives supported it. Test it on you sysem. Modify config files etc. Some people wanted graphical tools for enabling it. Today, it is automatical enabled if you hardware supports it. Not only do you not need to mess around, you don't even need to know it exists.

There will always be people who prefer to use the terminal and find it faster. And these people will mostly give commands to solve problems on forums, even if there is a simple graphical way to do things.

There will probably always be things that require a terminal, but I imagine these will be things that most people dont need or want to do. Things like programming, server administration, debugging etc.

I have install linux for people who have no interest in how a computer works, and would never use the terminal. Doing an install will probably always require a bit of knowledge and interest (but everyone knows a nerd :-) ).