Yes I agree, the UI is a mess and they have made some bad design decisions. One time I was on train with my network connection dropping constantly and Thunderbird was giving me a model pop-up dialogue each time! It still gives pop-ups for updates. But it's open-source, relatively stable and reliable, and that counts for a lot.

The sad fact is that there is simply no money in building email clients. Email is a legacy technology and arguably no longer fit for purpose. We need a new standard and that's not something that BigTech has delivered yet.