Linguist, it appears that the jury is still out regarding whether combination therapy with a biologic and immunosuppressant should continue long term. There is a concern regarding lymphoma with long term therapy with azathiopurine. But, the key is if you are in full remission before considering stopping one or another treatment. The immunosuppressant makes it possible to sustain lower doses of Remicade over time, by suppressing anti-Remicade antibodies.
I was able to find this article. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5479343/
Your azathioprine dose is higher than mine. I only take 50mg daily, and have since the beginning of my treatment. Perhaps because I am also on treatment for my arthritis in addition to IBD, there has been no discussion of stopping any treatment. It has been 18 months for my combination therapy.