Funny. My grandpa has been using Thunderbird and Libre Office for years, and he never realised it until recently (and he uses it a lot). He recently had an issue for the first time and asked me as he was trying to fix it with Microsoft but didn’t get anywhere, and I had to break the news to him it wasn’t their product.
I’m not the one who set it up for him btw. But whoever did so made it look as much as to make it easier for him to switch. Which worked as he had no clue and thought he got some free version or so.
I do also use it, but my setup isn’t Microsoft-like per se. I’m rather happy with it tho.