You do realize OS X is a flavor of UNIX kernel that runs both Linux and OS X. In a sense, using Linux would be analogous to using OS X; heck, similar even.
No.
Mac OS X and the various Linux distributions** are two examples of a huge family of "Unix-like operating systems" that share some concepts with the original Unix operating systems and are partially source-code compatible. The kernels are completely different (in fact there's a holy war in the Unix community about kernel design, and OS X and Linux are pretty much on opposite sides).
Mac OS can't run Linux software. What it
can do is run software written for Unix-like operating systems (including Linux) that has been re-compiled from the source code after making a number of patches to make it work properly with MacOS. Also, a lot of Linux/Unix software with a graphical user interface needs the X Window interface and the Mac implementation of that is pretty clunky alongside the Mac user interface.
Linux-to-MacOS porting can be a
lot easier than (say) porting software from Windows - partly because Linux/Unix programmers tend to think more about cross-platform issues than the Windows-only world - but it can still be a non-trivial job.
Going the other way, Mac OS includes many proprietary Apple libraries and application frameworks (including the whole Mac graphical user interface, and a lot more besides) - it
is possible to write code for command-line tools and suchlike that runs on both Mac OS and Linux - or even more sophisticated stuff if you stick to cross-platform tools and libraries, but the majority of native Mac applications would need a major re-write to run on Linux.
From the point of view of most users, the Unix parts of MacOS are irrelevant and switching to a "traditional" Unix-like OS would be switching to a completely unfamiliar system.
*** If you want to get pedantic, (a) Linux refers to an operating system
kernel (also used by Android and ChromeOS) and complete operating systems like, say Ubuntu or Fedora include lots of stuff that is not technically part of Linux and (b) Linux is
not Unix, but MacOS
is Unix, because its certified and registered with the group that own the Unix trademark.
However, a more curious question to the OP, why Linux? Why not another flavor of OS X?
Because they have an old Mac that isn't supported by any version of MacOS newer than High Sierra.
