Fashion in chess openings is a an everchanging process. The elite players are those who dictate the fashion and what they play is played on all levels afterwards.
On the surface it may appear that one line is replaced by another for no apparent reason. But there is always a reason and that reason may have nothing to do with the line that has been abandoned. Very often the elite players «trust each other» when it comes to opening ideas and when one of them comes up with something (or changes a line) the others pick up from there, do their own work and start playing the same line.
Let’s take for example the popular line with 5 Bf4 in the Queen’s Gambit Declined.
One elite player who constantly plays the QGD is Hikaru Nakamura and for many years he exclusively played the line with 6…c5, the «old» main line – first he sort-of resurrected it and then continued to play it successsfully.
However, this year he changed the line in favour of 6…Nbd7. Mind you, theoretically speaking there is nothing wrong with the line with 6…c5, but Nakamura felt it was time to change. So he did.
What happened next is typical – everybody who played the QGD started to play 6…Nbd7. Usually when a player has success with some openings the others follow suit (and Nakamura is currently very successful in the online events taking place) – for example a player like Karjakin who can play the QGD but it wasn’t his usual repertoire, started to do so and made it his mainstay defence against 1 d4.
Even the World Champion isn’t immune to these influences. He also used the QGD on several occasions, though Carlsen is a player who likes to go his own way – so after losing to Nakamura (!) in the 6…Nbd7 line he switched to 6…b6 in later games.
After 6…Nbd7 the move 7 c5 is the most critical one, «punishing» Black for failing to play the move himself and grabbing space on the queenside.
Now opinions differ. Kramnik always played 7…Nh5, immediately taking White’s light-squared bishop. Karjakin does the same as did Carlsen in the above-mentioned game against Nakamura. After 8 Bd3 Nf4 9 ef b6 10 b4 a5 11 a3 c6 12 0-0 the critical position arises.
Black can play 12…Ba6 or 12…Qc7 here.
Nakamura on the other hand prefers the move 7…c6, allowing White to keep the bishop.
Black will again continue with …b6, …a5 and …Ba6, exchanging the light-squared bishop.
The difference between the two plans is that in the former there is one pair of light pieces less – the exchange of the knight on f6 for the bishop on f4 makes it a bit easier for Black who has less space, but White has stronger grip over the e5-square. This makes it possible for White to play Ne5 and after …Ne5, fxe5 his space advantage would extend to the centre and kingside. The latter plan on the other hand leaves Black with 3 light pieces but White has less grip over e5 and Black has the idea of …Bd8-c7 in order to exchange the dominating bishop on f4.
Based on their choices it appears that the elite players haven’t decided on the better of the two plans. This means that both are equally playable with approximately equal merit. This furthermore indicates that the line will continue to be played until one of them decides it is time to change again. Then the process will continue with the next variation.
I have always found it fascinating to follow these changes and developments even though for anybody outside the top 100 or so these mean little. A sound repertoire is mostly immune to the changes in fashion and a classical opening like the QGD can be played almost without paying attention to what the others are doing. As they say, the classics are forever.