(Googler, opinions are my own. I don't work on any of these products)

Hangouts was born out of Google+, and that merged with gchat. They rebooted it's UI a few times through all of this. Over the years they put more and more features onto it (like sms app integration, phone dialing, and many other things). If you used Hangouts as your sms app, you could txt from hangouts within Gmail. I think it became a franken-app that was just too hard to add to or improve anymore.

Duo / Allo: These were to be the consumer focused products. Allo obviously failed (I actually liked a lot about the UX of this app, I'm really sad it died). Duo seems to be a Facetime competitor. It focused on 1:1 video chats for a long time, with lots of fun little thing being added on (like the animation overlays), and leaving video messages. The UI is dead-simple, click a picture, get a video call. Tech wise there are a couple cool things. [1] it is end-to-end encrypted. [2] It is optimized for mobile. That means it adjusts very quickly to bandwidth and latency changes, which is great for cellular networks. Note that it has some group chat now, but only on mobile. (there is a desktop browser version, but that's still only 1:1). It also has integration with Google's phone app on android. EDIT: few other fun things: Duo and Allo can be used without a Google Account, only a phone # is needed. Allo had a web app so you could send/receive from your computer, this tech ended up being the foundation for the sms Messages app web client (both were available for a while and the UI and behavior was exactly the same [2.1]).

Chat: obviously a slack competitor. It has bot integration, threads, and probably some other things. As a daily user of this, I will say message delivery and notification are much better than what old hangouts used to provide (when you are on desktop and mobile a lot, notifications were sometimes not great. It seems better with Chat).

Meet: stand alone video. While it seems very similar to the old hangouts video product, I feel like it's been improving quickly and there are lots of smaller features that I enjoy. When a meeting gets to a certain size, it defaults to muting new people that join (this is minor, but proven to be nice). Desktop sharing on it works rather well (at least on chromebooks, I can share a tab, a window, or my whole desktop. Sharing just a tab is nice so I don't accidentally share my email with outside people for example). For enterprise, you can use Meet to do live streaming to really large crowds[3]. There will be a normal meeting for presenters, then everyone watching gets a youtube like video player, and this supports up to 100k people. It assume better bandwidth, so it doesn't degrade on low bandwidth as well (or so I read, but maybe it's better now). Meet has lots of other enterprise features[4], and has a focus on meeting rooms, hardware, UI and things that go with installing this product into rooms. They also have Jamboards, so you can do digital whiteboard that can be shared with everyone on a call.

All 4 of these products got to start somewhat fresh tech-wise, and you can see that they are able to move rather quickly now, which is something classic Hangouts just wasn't allowing teams to do. Was it the right call? /shrug. For GSuite, there is at least Chat+Hangouts interop, so there is a migration path between those products. And I believe it was announced long ago that Meet+Chat would replace Hangouts for consumer, just not clear if/when (talked about Jan 2019 [5]). They were meant to be consumer vs business focuses, which seems to allow PM/UX not to fight about which features and designs they should move towards. The old hangouts mobile app had TONS of buttons in the past, and it could get confusing for non-tech savvy people, and I think they wanted to avoid this.

[1] https://support.google.com/duo/answer/9280240?hl=en

[2] https://en.wikipedia.org/wiki/Google_Duo

[2.1] https://www.theverge.com/2017/2/23/14721280/google-allo-chat...

[3] https://support.google.com/meet/answer/9308630?co=GENIE.Plat...

[4] https://roomdesignguide.withgoogle.com/equipment-locations/i...

[5] https://www.theverge.com/2019/1/22/18193303/google-hangouts-...