This is exactly the point why I hope that other projects like pinephone (Comparing specs of upcoming Linux phones) also are a success. It is not an exact copy but it’s sharing crucial parts like being able to install plasma and gnome mobile.
As Dr.Lambda I also think that GNU/Linux phones will not become mainstream. Let’s hope they can survive. More companies creating different linux phones (see above) diversifies the offer to clients which means some can buy a pinephone as it’s cheaper others a librem 5 as it has more freedom and better specs and maybe soon (?) other phones (as purism wants to open source a lot of things it gets even easier for others).
So more devices lead to more clients. With more clients companies in total can invest together more money in the operation system / apps. With more clients there will also be more apps ported with libhandy or kirigami by linux devs.
Only future will tell us if we are with a white elefant, but there are at least already other white elefants 
Also if you project this view to linux in general you would see that Linux as operation system is also surviving for the desktop while the usage in percentage is very low, but:
- Linux is used a lot in universities where informatics is shown - which is a perfect pool for new enthusiastic developers
- Linux is used a lot for servers, things done there are shared with the desktop
- There are companies with a business also using desktop linux (look who pays your fav distro / desktop-enviornment developers) - they don’t have the highest market share, but they survive
What does survive mean? Do you want to get a higher market share so that everone goes to this platform or is it fine for you to stay niche? I would say survive means that we still get a new kernel, software updates, security updates until the hardware breaks. It does not mean that WhatsApp, Skype, Ubisoft and your local bank create apps for the librem 5
So I conclude:
- Purism did the right choice by upstream / mainline everything (share so that everything is maintained easier and can shared and shared back with servers, desktop, …)
- There are already other projects -> If they are also a sucess there will be more total clients
- The total clients have to be enough so that the companies or the community let the device “survive”