@roni_chessman:
1) books
I do not completely disagree with you on the topic. I would consider something along the lines of going through the entirety of a 300 pages book vs the french a waste of time as well tbh. If you are playing the french as black (or ruy lopez or w/e) you can look at some parts once or twice to get a better understanding of white's ideas, its not too bad imho. Otherwise..you're right on this one.
I know where you're coming from when it comes to the tourney example. Nevertheless you need some sort of repertoire against these openings regardless (I'm not playing 1.e4 but its exactly the same for 1. d4 2. c4). Not saying you should always go for the main lines but some knowledge of YOUR lines + strategical/tactical motifs in the early middlegame positions are beneficial I would think!?
2) opening study
a) usefulness of opening study in general: I know I got an unpopular opinion on this one - but I don't think it hurts to study openings above a certain lvl (not even that high, let's say 1800).
Once in a while you get large advantage / winning positions with white or slightly better positions with black straight out of the opening which is quite substantial on its own. But you are also training your positional understanding in the process (even looking at stockfish lines, let alone playing through GM games in the particular lines). Not saying it's the only useful form of training, but it definitely deserves its spot somewhere imo.
b) main lines vs non-main-lines
It's a very interesting question and largely a matter of taste / fit of the arousing positions I guess. Some psychology is involved as well. You are essentially saying that people are comfortable and good playing their main lines / structures. I am not that sure. I often found people outright bluffing in their main repertoire and you could refute their lines with a bit of preparation. This is less true for very strong players (IM's + or so) but those have a very decent grasp of side lines as well. Below that level it's somewhat random, you can get a 1900 with very good understanding of the opening or an FM with 0 clue whatsoever (ofc it's more likely to be the other way around but still).
Are people "worse" or "easier to exploit" in the opening, the middlegame or the endgame?
TLDR When it comes to taking people out of their comfort zone I generally tend to agree IF they are really that good in their structures OR really as clueless as you say. Finding out what lines people play poorly is a part of the opening study as well, so you've done some work there regardless of your opinion on the opening study ;)
