I assume you mean IBM's IMS. I don't think it was replaced for lack of flexibility. AFAIKT it was replaced (by those who could replace it. There are many that

I assume you mean IBM's IMS. I don't think it was replaced for lack of flexibility. AFAIKT it was replaced (by those who could replace it. There are many that still use it. Like banks) because IBM didn't let it progress, basically froze it in the cobol era, tied it with their hardware, and continued to cash-in as long as they could. Unlike some sister comment, I think it was conceptually great... Only the tooling and ecosystem were/are terrible.