I'm not saying he is wrong for doing this - I know of many competing projects, but nothing more. Maybe there is a good reason not to join them. However my guess is not invented here which holds us back.
Just by chance I looked the other week into installing LineageOS on a slightly older LG but then decided it was too much hassle - although I have experience with Cyanogen, i.e. it would not be the first time for me to install a custom ROM.
All I use on my phone are maps, email, messaging and a browser. I can easily do without Google Play Services and I am sure there are many others.
He is french and in France there are many people thinking like him. It is easy to get press coverage and therefore should be possible to create a user base of a few 10.000 people with little investment.
That's just how it is.
Also, he's not duplicating anything, he's using LineageOS plus new stuff that will be open source.