There are lots of clients that have unified inboxes.
I could name other amazing clients too (like Airmail for MacOS), but will focus on just the best ones for Windows:
- Mailspring
- eM Client
- Mailbird
- Spark
These are in order (considering the best UI, Performance, Features, Stability) and all of them support a single Inbox.
There are a ton more clients, and I've tested them all. None are worth being on that list for several reasons I will skip for now.
Now, back to the real Outlook issue:
Outlook needs a unified Inbox. Without it, people with multiple accounts (work, personal, portal, etc.) find Outlook obsolete and unusable, so they have to switch clients.
By switching clients, you then lose the most important feature Outlook has, which is OS integration.
Now you lost the native calendar meetings including the flyout, you rely on connecting 3rd party services to handle the whole calendar integrations (and worse, if you have multiple sources), you have to figure out why the meetings are not updating/appearing when they change, and you have to work on workarounds while granting access to who knows what tool to make the sync happen. Then you have to live knowing all the Microsoft services are still running while you simultaneously run a bunch of apps (unnecessarily eating resources) to do the same thing.
Even worse now, where in the new upcoming "Outlook for everyone" now Microsoft integrated AI (Copilot) which is an absolute fantastic feature that you will never be able to use. Why? Because it still doesn't have an unified inbox.
You get the point? We need a solution. It can't be that hard. I'm a developer. Do you want me to join you and do it for free?