I am 30 years old, have Master's degree in Machine Learning from a CS department. I also have a job, which I have had for like 4 years, first as a software developer, now as a data scientist at the same company.
I have been thinking about PhD for a while now. Is it worth in the field of Machine Learning or statistics (though I think my maths might be too weak for the latter)? My prime reason to do it is that I think that my knowledge in the ML/stats field is not enough, so I would like to dedicate a few years to learning much more even at the expense of low income, and do some paper publishing to meet the quota of "strong publication history". Plus I like to do research, though I like it in industry setting more, since problems are more alive, if you wish.
My particular concern is the work placement after getting my PhD. Many people say that PhD limits the opportunities of a person holding it, but is it true if I already have some industry work experience plus going to apply for ML jobs, unless hype completely disappears in 4 years?