In the ongoing debate about ESL being a job or a career and success in life, my own position
was that there are objective indicators for success and not only subjective factors
determining the perception of "success".

I don't personally believe that there is an 'ongoing debate' as to whether or not ESL teaching is a career path or a job. I think most of us accept it as being a job.

Merriam Webster gives the definition of success as follows:-

Definition of success

1 obsolete : outcome, result

2 a : degree or measure of succeeding b : favorable or desired outcome; also : the attainment of wealth, favor, or eminence

3 : one that succeeds

With regards to the word 'success', you make an interesting point, but before I give you my own feelings on the subject, allow me to ask you a hypothetical question, particularly in relation to number 3 above:

Let us suppose that you want to join ISIS. You need to work hard; you need to undergo lots of rigorous training that may last several months. Once you achieve your goal and become a fully-fledged member, would you consider yourself as being 'successful'?