The only thing I see winning against Google and Microsoft is some hybrid, open source local + cloud suite of tools. Something better than Thunderbird, more like Obsidian. A combination of native and cloud, working well across all platforms, integrated with a full suite of other productivity/office tools, but most importantly open source. Because I don't want to trust my data to some other tool that will just dry up and disappear.
That's a steep gradient to climb. Really steep.