I hear ya. I bought Kiwi for $5 two years ago. Back then, not only did it have all the issues you point it, but it wasn’t even that stable of a program, crashing often. They were promising 3rd party integrations back then too, which I’m sad still haven’t materialized. Still, I never looked back. I would have paid $10 to google if they had made a native app for their otherwise free service, but they didn’t. It is better than Outlook, Apple Mail, or any of the other programs I tried.
I don’t like using the browser for my email, or really any other productivity tool — simplenote, wunderlist, evernote, dropbox, spotify, kindle, ms office, etc. Access through a browser is great in case I’m not at my main computer, but if I had to access everything through browser windows all the time I would be constantly frustrated and underproductive. I avoided using the superior gmail frontend precisely because it was only accessible through the browser. I don’t use Kiwi for any “new” features or “premium” features — I use it despite it’s lack of gmail’s native features because being able to access it outside the browser is a “killer feature” easily worth the $10. I’m glad it exists, and hope more people support it so that it continues to improve.