My thoughts are that your doctor is thinking that you are having side effects and that is why he is reducing the dose. However, I personally don't see the logic in it when there are other biologics that are known to be effective, plus they are more cost effective. They are more cost effective, not because they are less expensive. All biologics cost close to the same per month. But, only Remicade requires IV infusion, incurring a great administration cost (not necessarily to you, but to your coverage provider). The others are self administered. To me, it is a no brainer to switch to the humanized biologics, which are more convenient and less likely to cause antibody/allergic reaction. My rheumatologist never even recommended Remicade as an option for me, and I am glad for it.
Reducing your Remicade dosing is not a guarantee that you will suffer a relapse, as it is typical to stretch your dosing once you are in remission. Have you discussed changing drugs with your doctor?
My thinking also is that your arthritic symptoms are not a result of the biologic, but from your IBD, meaning the Remicade is not fully handling your disease. But, when you take double doses, you double your risk of side effects, so something to keep in mind for long term use.
Granted, the doctor's office loses money this way, but I doubt that is a factor, or at least I hope not!
Jan