As a non-Muslim, I've been learning about Islam and found it beautiful, but I'm puzzled by one question.

Assalamu alaikum. I hope you're all doing well. Recently, I watched a YouTube series called 'O Messenger,' which deeply explores the life of Prophet Muhammad (peace be upon him). As someone not very familiar with Islam, I was captivated by it-the series was incredibly well-made and truly inspiring. It opened my eyes to the beauty of early Islam and the Prophet's amazing life. After finishing the series, I delved deeper into Islam and discovered it consists of 73 different sects. As we know, many religions await a significant figure in the latter days. For instance, Christians await Prophet Jesus (peace be upon him), Jews await Prophet Moses (peace be upon him), and Islam also speaks of Prophet Jesus's return. What caught my attention was that one sect in Islam claims the Messiah has already come, which surprised me a lot. I hadn't heard of any other group making such a claim. I read quite a bit about this sect, known as the Ahmadiyya group, founded by Mirza Ghulam Ahmad in 1889. But what's been bothering me is something else. I've read that other Islamic sects consider this group false, saying Mirza Ghulam Ahmad is not a real Messiah because they believe he disputes Prophet Muhammad (peace be upon him) as the final Prophet. However, my confusion comes from this: Islam teaches that Allah has promised to completely destroy the legacy of any false prophet or messiah. If that's the case, why does Mirza Ghulam Ahmad's group still exist and even grow over 100 years later? Why hasn't this promise seemed to apply here? I'm trying to understand-does this mean the promise isn't true, or could there be more to it? It feels like a contradiction, and I see this promise as key evidence to either reject or accept his claim. On a side note, I've also read the Ahmadiyya perspective that they do believe in Prophet Muhammad (peace be upon him) as the final Prophet to bring a new religion, viewing Mirza Ghulam Ahmad as a reviver of Islam's teachings, not a founder of a new faith. This is how they explain it. I'm not leaning towards any sect; I'm just sharing what I've read and wondering about this question. I'd really appreciate hearing thoughts from Muslims on this. JazakAllah khair for taking the time to read.