I think it's eventually inevitable for non-human intelligent life to emerge. We were the first species. We think we are kings of the world. But we won't be in perpetuity.
This can happen through multiple ways.
The first way is through simple genetic drift. Think of humans colonizing the galaxy and living in isolated populations for millions of years. Eventually we turn into different species. So maybe there is one hominid species right now, but in 1 million years it will look way different.
Another way is through a human deliberately creating nonhuman intelligent, but still biological life. Think of people wanting their pets to be smarter, furries wanting to turn into half animals, or just experimentation for science like in this instance. Maybe 99% of society is repelled by this, but if you give the 1% which desires this enough time, they'll find an opportunity to bring such life into existence. However great the taboo, eventually someone will break it. And once there is nonhuman intelligent life, there will be people who want to protect it.
A third way is through humans creating silicon based life, e.g. to upload rich people to the cloud, letting them live there like gods. Maybe if you initially build such a system, you make the experience as similar to human experience as you can, with all the unpleasant parts of it removed, but the people it simulates may eventually grow tired of interacting with the fleshy human society that they form the elite of and instead focus on interacting with each other instead. Maybe they'll then accept changes in programming, allowing human brains to be merged, duplicated, for short times, or through random mixing, etc. I think computers allow a great deal of things and likely we'll try them out sooner or later.
To summarize, I think we are at the start of a cambrian explosion like event. Think what happens if you are the first plant that can grow on land. You'll quickly fill all the lands and your numbers will have no limit. But by the time you have done that, you are a collection of different species instead of just one because your siblings had to adapt to the different environments, and the species themselves are competing with each other.
Regarding aliens, I doubt we are alone in the galaxy. I'm sure there is independently developed, intelligent, life somewhere. But likely by the time we encounter them, we'll maybe already be a cluster of multiple intelligent species.
Or idk, maybe not. Maybe there is only one hominid species because we are so utterly racist and have eliminated anything that was even a little bit different from us :).