My vision:
- Costs are money and time.
- Risks are 1) Fail your PhD 2) Fail your other diploma 3) Fail both. Its imply loose of money, time, career perspectives and maybe some self-esteem.
- Expected benefits are 1) Intellectual stimulation 2) No regrets 3) The small possibility to completely change your job domain.
For the record, a friend of mine try a similar situation: He was in its last year of business bachelor and start a part time history degree. He ends up fail its final exams in business bachelor and completely give up the history classes. He re-try the bachelor business next year and succeed. In other words it was a complete fail for him and he was not lazy or stupid but any human has its limits.
If you really hesitating try to do your own list of positive vs negative aspects, it might help.