There is nothing which Apple Intelligence can do that a hypothetically evil Apple couldn't have done before, given sufficiently treacherous code in their operating systems. Thus if you use an Apple device, you're already trusting Apple to not betray you. These new features don't increase the number of entities one must place their trust in.
Whereas with apps like Gmail and WhatsApp on an iPhone, you must trust Google and Meta in addition to Apple, not in place of Apple. It doesn't distribute who you trust, it multiplies it.
I still think it's a big difference between trusting existing OS'es and apps, which are under scrutiny by hundreds of security researchers and thousands of security nerds all the time, and willingly sending away all your data to a party who promises they will treat it well (I know it doesn't work like this in this case, but just for the sake of argument).
In essence, what you're doing is training an assistant to learn all of your details of your life and habits and the question is if that "assistant" is really secure forever. Taken to the extreme, the assistant becomes a sort of "backup" of yourself. But yeah it's an individual decision with the pro's and con's of this.
Whereas with apps like Gmail and WhatsApp on an iPhone, you must trust Google and Meta in addition to Apple, not in place of Apple. It doesn't distribute who you trust, it multiplies it.