How durable is steel? Would there be any advantages to making it more
durable, or more easy to obtain?

In general steel is very durable however a few factors need to be considered.  

Firstly there are a huge number of different types of steel.  Steel is
an alloy and as such there are a huge range of blends that can make
steel.  the most common definition of steel is a metal alloy that is a
mixture of Iron and Carbon with carbon generally making up less than
5% by mass.

The main factor in deciding the strength and/or durability is the
concentration of carbon within the alloy.  it is also possible to add
other elements to change how the steel performs.  the most well known
is to add chromium to make Stainless steel.

There are also a huge number of physical processes that can be applied
to improve the physical properties.  these range from heat treatment
to work hardening.

However possibly the greatest factor deciding the durability of steel
is the type of corrosion protection chosen.  Corrosion is caused by
the iron in the steel oxidising with the air in the prescence of
water.  This can be prevented through coating or by mixing in other
metals i.e. chromium added to make stainless steel.

In conclusion steels are a very diverse range of metals whose physical
properties can be tailored to a certain range of specific needs.

Sorry this has been a bit vague but There are lots of different types of steel.  

Mike

In addition to what mike said there is also fatigue life to consider.
If you had a process to further extend the fatigue life of an already
developed alloy then it would be very desireable. Fatigue life is the
number of cycles of a periodicly appled stress that a material can
withstand before failing. With steel it is possible that this will
never happen--what is said to have an infinite fatigue life, when the
stress level and number of cycles are below the endurance limit.

So like I said, if you could increase the endurance limit (or increase
the fatigue life) on the material without compromising its material
strength it would allow engineers to design more agressively and it
would be very advantageous.