I don't personally find Thunderbird to be bloated, especially compared to e.g. KMail (which even a few years ago was still buggy and unstable for me). "Complex" might be reasonable but I expect a lot of that comes from email protocols themselves being convoluted and weird. In fact, I think email's own complexities are what destroy most peoples' motivation to make a shinier, more modern tool.
I will say I do have a massive, single gripe about Thunderbird, and while it's a trivial thing it's also - and in part because it would be so easy to fix - massively infuriating: the default keybindings for archiving, deleting, marking read emails etc. are, astonishingly, single characters. I've lost count how many times I didn't correctly highlight the filter textbox and started to type a search query, which instead had the effect of randomly archiving, deleting and marking read the top N emails in my inbox before I notice. This is literally why we have control characters, so I find it incredible that this is not only the default behaviour but unchangeable without a plugin [1]. I still find the tool very useful in every other way but I can't believe something so obviously wrong has not yet been fixed after what must be at least 20 years of this software tool being available.
[1] https://addons.thunderbird.net/en-GB/thunderbird/addon/tbkey...