This happens often. You create a community website. People join. Many people follow. It gets popular. You get rich. A few members become unhappy about something. You don’t respond fast enough. Those members create a new community and a large majority of people follow them.
The biggest threat to your online community isn’t from external companies and organizations. It’s not from a rival planning something similar. Outsiders are usually irrelevant.
Overwhelmingly, when an existing community is overtaken by another, it's a former member of the community whom launched it.
Only your existing members have the contacts and reputation to bring a lot of people with them when they move. The biggest threat to your community is an unhappy member. Not an outsider.