I mean I think the reason that line is not so popular is that the plans in the main line are clear.  I mean basically to prove a real advantage you need to grab space in the center or get some seroius initiative and I don't think the d4 line does this, not that there's no advantage, just maybe not as big.

3.d4 was extensively tried out in the early 90's. The match Timman-Yusupov and some Linares games involving Kasparov, Anand and Ivanchuk were critical.

Now 3.Ne5 is being explored. With 3.Ne5 white tries to prove that  the Ne4 hinders black with c2-c4, in the 3.d4 variations after Nd7 Nd7 Bd7, black has an extra (although not terrible useful) tempo...

Anand tried an exchange sacrifice against Ivanchuk in the 3.d4 variation, but the line is thought harmless by Jangava in his Gambit book.

1.e4 e5 is looking terribly solid these days....