I would think of using 'Frenchie' to refer to a person as being affectionate banter. Like 'Yank' for Americans or 'Canuck' for Canadians. It's not incorrect, but would be inappropriate outside of an informal context.
French people have 'rosbif' to refer to the English and Australians have 'pom' or 'pommie'. You wouldn't call the prime minister that at a diplomatic event, but it's not offensive to call your friends that.
I'm using the words 'grammatical' and 'ungrammatical' in a linguistic sense; human languages are subtle and fluid, and one doesn't have to be far along the sliding scale between 'doesn't speak a word' and 'well-educated native speaker' to be understood. We speak of 'broken' English when somebody is able to be understood but hasn't fully grasped the language yet; using demonyms incorrectly is a subtler flavor of the same thing. For example 'no come here' -> 'no entering' -> 'no entry'
It's not completely wrong, it will be understood, but it is ungrammatical and a clear marker that the speaker is not native, similar to getting adjectives in the 'wrong' order ('a big tasty sandwich' sounds more natural to a native speaker than 'a tasty big sandwich', even though the latter makes sense and will be understood).
Demonyms for historical neighbours of England have irregular forms when speaking of a particular person from there. Scotland has 'Scot' and 'Scotsman'; Wales has 'Welshman'; Spain has 'Spaniard'. Other countries indeed need a second word, such as 'person' or 'citizen' ('a Chinese' sounds offensive to me; I would say 'a Chinese person' in all cases). The only country I can think of where using a bare demonym is grammatical when speaking of a single person from there is Germany with 'a German' - probably because it has the suffix -man.
Edit: A sibling comment pointed out that 'an American' is grammatical, and thinking about it, I think the suffix -an is what makes bare demonyms grammatical - you can say 'an Angolan', 'a Laotian', 'a Peruvian', 'a Moroccan', etc, but wouldn't say 'a Thai', 'a Swedish', 'a Sudanese', etc.
Loper-os seems to suffer from the same problems as a lot of "unix hater"/lisp weenie types: while he does point out real flaws in the computing of today, he seriously lacks in a competent, coherent, non-vaporware alternative. His blog is now nothing but bloviating about bitcoin
>We now use smartphones and tablets most of the time, since they are much easier to use. ... With people switching to mobile devices for mundane tasks...
Who are these people and for what tasks? Afaict, except for email and Facebook, the promised death of the PC is extremely overblown. In a professional setting, even major phone addicts still use PCs or laptops.
French people have 'rosbif' to refer to the English and Australians have 'pom' or 'pommie'. You wouldn't call the prime minister that at a diplomatic event, but it's not offensive to call your friends that.