Is it worth paying for Kagi when it's future is uncertain? Are there any real competitors for paid search?
Kagi is already profitable and organically growing, so at least from a financial standpoint the future is certain.
If Kagi goes away, it should be relatively easy to switch to an alternative :-)
Is it worth investing early in better things so they have a chance?
Yes.
The worst that can happen is the better thing goes away again, and your payment goes away too.