If you are doing something you love than this would certainly apply to you.   Remember that you have this job for a reason.  Chances are you are good at it and others recognize that. 

Be confident that there are not a lot of people that can do what you do, but certainly don’t be ignorant or arrogant about it.  Even though all of us can be replaced… never let that consume you.  Instead, let it motivate you.  Scared animals run and hide.  Winners always see opportunities.  ( be a winner unless you want to be consumed by the lions of your industry)

They can hire someone else, but there are only 2 outcomes if they do: 1) they hire someone who is awesome and will do the things that you should have learned to do. 2) they hire someone weaker than you. (in this case, they get what they deserve) In the meantime, be open minded, willing to learn and always work ahead so that you have extra time to apply towards your ongoing education. 

Never step learning, (unless your plan is to get phased out) When you feel like you have paid all your dues…this is about the same time you put yourself in a replaceable situation.  Keep paying them.  Work is designed to never be finished-neither is your education.