Either they're okay with a switch and it's easy or they are not open for that and it's impossible to change their mind.
Pretty much nobody I know wants to switch to Signal or any other messaging app. So it's SMS communication with them because I definitely won't install WhatsApp.
They know the names of their customers but they don't know their search queries. There's the privacy. At least that's what they said. Since Kagi isn't Open Source it always comes down to trust.