It seems to me that after the popular move order
[fen ""]
1. d4 Nf6 2. c4 e6 3. Nf3 d5 4. Nc3 Be7
the most popular option - at least at top GM level - is currently 5. Bf4. Yet, back in the 1980s and 1990s, 5. Bf4 was not considered a particularly challenging way to fight the Queen's Gambit Declined. Every strong GM was playing 5. Bg5 (or the Exchange Variation). Who revitalized 5. Bf4, and why is it considered a strong / challenging move nowadays?
One of the possible reasons why 5.Bf4 has gained popularity at the very top-level, is because white seems to have difficulties obtaining an advantage after 5.Bg5.
The critical variation is 5....h6 6.Bh4 O-O 7.e3 Nbd7 8.Rc1 c5 9.dxc5 Nxc5 10.cxd5 g5!, which occured for the first time in Wojtaszek-Kramnik (2015). Moreover, black can also play 9....dxc4, which happened in another famous Kramnik game: Matlakov-Kramnik (2015). After 8....c5, white's main alternative is 9.cxd5. In this line too, there is an impressive Kramnik win: Salem-Kramnik (2014).
To answer the question "Who revitalized 5.Bf4?": Kramnik, by posing some questions as black in the 5.Bg5 line.
[StartPly "8"]
[FEN ""]
1.d4 Nf6 2.c4 e6 3.Nf3 d5 4.Nc3 Be7 5.Bg5 (5.Bf4) h6 6.Bh4 O-O 7.e3 Nbd7 8.Rc1 c5 9.dxc5 (9.cxd5) Nxc5 (9...dxc4) 10.cxd5 g5
