The following are my opinions on the subject, on not someone else - so burn and flame on me...
Evidence A - We need unified messaging, not splitting
Multiple message apps - hangouts, duo, what else...is the opposite of business and consumer request. Our digital life is a mess of different applications, multiple todo lists, whatever - please have all of ours sms, IM and whatever in the same place. Hangouts could do that - nicely - but
Google Alphabet is taking a different direction. Tell me - do you really want that?
Evidence B - You remember BYOD, right?
BYOD means a blend of professional and personal life in the same device - my (GMail) account for Android and personal data Sync (and backup), and in the same smartphone also the professional info, the bothering appointments, the infinite work to do list - a curse for someone, a blessing for many: no more "I have different schedules on different apps", but our only, central agenda, no more (unwillingly) forgetting of personal events while planning professionally.
Evidence C - Google is doing business, too - but wrong
This is by no means a new one, but Google is doing big business right now, and I could name a big, big corporation making the switch to BigG in six months. The issue here is that they are taking a wrong path with the whole messaging part. When I had to join them for a call on Skype for business, all I had to do was to tell my name after clicking the link, right now I have to wait for someone to authorize me joining an hangout for which I have a regular invitation - something is clearly wrong here. And they are asking the same people not to use hangout for IM on the phone. Crazy. I see people using Whatsapp for IM purposes, even for work.
Evidence D - You could beat all of them, if you wanted to
Think about it - whatsapp? More or less the same. Add the persistent chat, and here you are. Slack? Build team options, and channels, and you have that too. FB Messaging? Why add another option if you have a decent one already in your phone, in your browser, in your car, in every damn android system you could own.
In the end
Please, please, hire someone as a strategist for the messaging platform(s) - heck, hire me - I could do no worse than you are actually doing! But put yourself in a clear and defined position (and don't let messaging go the way of the dodo.