It isn't about the hardware, and it also isn't about PureOS itself. It's about people caring enough to build an entirely new mobile operating system which actually respects your privacy and gives you tools to ensure it.
I'm really amazed at what they got running in the few years this project existed. Of course the phone won't be some spectacular mobile gaming machine, but that's not what I paid for. I paid for a privacy-respecting, secure phone. I paid for getting Linux to run on mobile OSes. That's what is important to me and many others.
Same here. I'll gladly downgrade some specs if it means I can run all free software on my phone. Trying to get google off of an unlocked android phone is still an uphill battle. Root access out of the box means I can install the software I want and remove software I don't (a novel idea).
The Purism team has accomplished an impressive feat with the Librem 5. Porting GNU/Linux onto a smart phone is a huge win for free software. I hope the transparency and upstream patches continue long in the future.
When I pre-ordered the phone, I also saw it as donating to (or at least contributing to the development of) open-source software in a way -- Purism is a for-profit company, but they've been doing a lot of software work on this phone and they've been pushing the changes upstream, too. I highly recommend looking through their blog (https://puri.sm/posts/), especially the Librem 5 [month] Software Update posts for the work they're doing.
It's worth mentioning that they have a strong commitment to their social purpose, not maximizing profits, which I really like. Read more about it on their webpage: https://puri.sm/about/social-purpose/
It's better because it uses mainline open source Linux drivers.
Your Android phone has drivers written for a specific kernel version. The userspace software can be kept up to date for a little while by making it work with the old kernel, but when applications eventually start to depend on newer kernel features it's time to chuck the phone in trash and buy a new one.
How this is better than installing lineage Os on oneplus 3? -- https://download.lineageos.org/oneplus3
I understand anyone who wants privacy need to pay extra, but is it really justifiable to pay this much for that?