I submitted a mathematical paper to a good journal specialized in geometry and topology. By good I mean that there is no doubt whatsoever about its scientific standing (e.g. Scimago places it consistently in the first quartile within its subject category).
Now, the problem is that I submitted the paper more than 19 months ago, and yet I have not received any feedback. The journal does not use an online submission system, so to inquire about the status of the submission I need to send an email to the journal's email address.
Approximately six months ago I was informed, upon request, that both referee reports had been received, and that the paper was pending the handling editor's decision. Last week I emailed the journal again (putting the Editor-in-chief on copy) and asked again for news. I was informed that the status has not changed.
I am wondering how long more I should keep waiting before withdrawing the paper. I do not want to withdraw it, knowing that a decision might be close, but at the same time I am getting really frustrated. What would you do in this situation?