If you as a surgeon and dont go to training , to keep you up to date, your old method may be hazard to the patient and you are not even allowed to operate.
Second we are talking about the french not a less known opening, if you study the french you probably know the ins and outs. Real study isnt just learning the moves but learning the poisitional and strategical ideas.
We are not talking about an unknown opening, the french is well known.
Ofc even grandmasters lose to old lines, even Kazimdzhanov used some old lines from someone who knew that stuff already. Does it mean you can just duplicate it and just take any random old line??? Besides people even lose to the grobe now and then, do you now want to play the grobe?? because its a marvelous opening??
Than there is the computer evaluation, what does 0.3 mean?? It could be an overwelming positional advantage, it could be a position where there is no life in your position and the only one with winning chances is the opponent. Not to mention who knows if the 0.3 is even accurate, maybe its often way bigger than you say, also not to mention that computers dont understand openings that well, or even some closed positions. Even Aronian played lines his computer said were not good.
I cant really judge the danish gambit, but for a weak player like me it seems black gets a really good game if he plays e5 and there is not much fun for white, even though its not worse position.
And all your other assumptions are incorrect, you dont learn the basics with old opening books, you learn incorrect and obsolete information mostly.
Do you remember Capablanca saying: i dont read opening books, they are all full of errors and even losing.
So you only study the best lines possible, the new grandmaster theory that gives you the .3 computer edge. Then what do you do when you are over the board and some body pulls out their pet project they love like the Danish or some fried liver junk or whatever. These are "bad lines", but can you win against them every time. You can't say that you would win every game against some old line. Because opening theory only goes so far. A line may give you a so called advantage, but you still have to be a great player to take that to consistent victories. I've lost many games to "outdated lines" so have you, so has everyone else that is not a top tier master. And even they can have trouble. I think the annoying thing about your argument is the idea you think it's wrong to learn "old lines", and that some average player will just destroy that old school crap. Pure nonsense. And I post this fully aware of the response that's coming. Someone posted this:
he is always right and will always have the last word.
Why does anyone study history?
Does a surgeon perform brain surgery before learning the head bone is connected to the neck bone?
I'm gonna win the tour de france, and I never rode a tricycle or had training wheels.
"How come can't get into Harvard?" said the eight grade drop out as he opens "Godel,Escher, Bach" for some casual reading.
Hey, I'm gonna land a plane on an aircraft carrier, glad I didn't study lift and drag and just went straight for hitting the third wire.
Mmmmm. a tasty souffle sounds like the bestest hardest egg dish to make, who needs to know basic cooking skills.
I could go on forever. It seems obvious. If we ever play you'll smoke me with current theory, but I won't be anywhere near those lines. As won't most of the people you play chess against.