This is going to be bad. Regarding the iPhone:
It will open the door to apps asking me to install their own stores to access them. That will be inconvenient.
It will open the door to apps charging me through methods other than Apple’s subscriptions. That will make it harder for me to cancel.
It will open the door to malware on the phones of my less technologically capable relatives.
On the other hand I will be able to install pirated Spotify and YouTube much easily. I currently have to use AltStore, which is somewhat annoying.
I’m not even going to comment on interoperability between messengers. It’s simply absurd, no matter the way I look at it.