In the Bart Ehrman introduction to the New Testament, he talks about this and says something essentially similar, that Jesus was just one of many miracle workers known at the time. I can't recall the name of one of the other men given as an example, but the idea of healers and miracle workers wasn't unique to Jesus' context.

So essentially, I agree - I'm not convinced by the claim that the stories about Jesus' miracles were entirely original and 100% factual as they're written.

However, I'm also not convinced that this means there can be no miracles, or events and experiences that appeared to be miraculous, or events that were miraculously serendipitous. Nor am I convinced that admitting the stories of Jesus borrowed from other religions necessitates admitting there was no Jesus, or that Jesus wasn't as significant as is claimed.

Humanity speaks from its context, we tell stories using frameworks that are familiar to us. Those of us coming to the texts of the New Testament come with a whole bunch of assumptions about truth, fact, and a number of other concepts that the ancient people wouldn't have made. We separate categories that weren't separate to them and make distinctions where they don't. We care so much about the connection between fact and truth, whereas I think for many in the ancient world (and even today) the "truth" of something was separated from the "fact" of things, depending upon the topic.

TL;DR: I think it's pretty obvious that other religions and philosophies informed the writing of New Testament and Old Testament Texts, and the religious expressions of both Judaism and Christianity. I think that other religions and philosophies continue to inform the way we think and speak about our religions today, they shape our theology and practice in significant ways. This doesn't seem particularly profound to me, and I don't find it problematic from a faith perspective.