No, WhatsApp does not bring the same features. WhatsApp is very different. You can react to messages in iMessage, which is just one among other features WhatsApp doesn’t have.
The fact their choice leads to needless exclusion of people who would otherwise be their friends.
I also have big trust problems with Facebook products. Apple makes iMessage to sell devices, what does Facebook do with What’s App? There have been reports in the past that installed Facebook, Inc., apps were sending analytics from devices on other apps people were using, pretty much like a Trojan horse. I don’t want any installed Facebook related apps on my devices because Facebook lost my trust long ago. While supposedly What’s App is “secure,” the entire history of Facebook has been littered with “oops, you caught us and we are really really sorry this time.”
When I try to use a "share" action for example, I get my contacts from the phone and Whatsapp at the same level. My list of contacts actually has a merged view of all my contacts stored on my phone, Whatsapp and other services. Contacts on multiple services are merged appropriately.
It works just fine.
This isn’t stupid. It’s the natural consequences of tech that doesn’t conform to real human behaviors and social norms.
I agree with power of defaults though. Only reason iMessage took off, and so did other Apple services.