They have issues with:
1. Memory usage and cpu usage (you're running a browser to run your application)
2. Platform look and feel (doesn't exist in javascript)
3. Platform and windowing behaviors.
I agree with the other commented to you.. if it changed to electronic, I would drop it as well.
This is a different target audience to the people who typically use Thunderbird at this time.
For some people Apple Mail is enough.
Plenty of people I know use that at work, but email is really nice, and I love the features Thunderbird has.
You live in a strange bubble.
Between github, SMS, social media, IRC & slack, video-conferencing, blogs, RSS, etc., email has become the medium of last resort. I still get a lot of email, but it's mostly automated notifications, and I usually have to log in somewhere to actually recieve a document because email is insecure.
Perhaps he/she work for a company that do not really know IT so force their workers to use limited and limiting platforms, but it that's remain for personal world then no, it's might appear to you a tech savvy person but that's only due to Kant-ian glasses, not reality.