Oliver had a bit of a down year last year compared to the previous two but he's better than an average starter by almost any calculation. As for our other DL, using PFF as a neutral reference, Rousseau had an 85 grade and AJ was at 80. That's way above average.
Yes, we signed Von for a talent ceiling upgrade and it blew up in our faces. We were a bit unlucky there. Diggs was also a talent ceiling upgrade, and for the first three years he was easily a top 10 WR in the league, if not top 5. You don't expect to get top 5-10 players at their position picking in the bottom half of the first round. So yes, only getting four years instead of five of team control wasn't ideal, and the dead cap is way less than ideal, but we got all pro level play for a good chunk of it, and have a top 40 pick incoming next draft because of the Houston trade. Characterizing the Diggs trade/era as a disaster is a total misreading of events imo.
Beane isn't perfect. He was the one who put us in cap hell in the first place, and as you point out, none of our top picks outside of Josh have become superstars. What we've had instead is rosters stock full of quality players, including those drafted in rounds 1&2, who are mostly above average for their position with a few guys like Taron and Milano who became top guys out of nowhere. I just don't see how anyone can make the claim that Beane hasn't been way above average.