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?