First of all, Nick Robertson doesn't play fourteen minutes per night, he averages just 12:05/game, which ranks 12th among TOR forwards. He's also not a consistent fixture on PP2 (just 0:58/game), and is definitely not dependent on the man advantage for his production (he has just 1 point there this season).
Over the last three seasons, Nick Robertson ranks 43rd in the NHL with 1.02 goals per sixty minutes of 5v5 ice-time, out of 709 players with at least 750+ minutes of 5v5 ice-time. Ryan Reaves ranks 421st, with just 0.43 goals per sixty minutes of ice-time. If we do just this season, Robertson ranks 114th, with 0.82 G/60, and Reaves ranks tied for last (out 624 players with 250+ minutes at 5v5), with 0.00 G/60.
Yes, Reaves plays less than Robertson (~253 minutes vs ~510 minutes at 5v5), but if you pro-rate Reaves' scoring rate of the last two years (we'll use two years, since he hasn't scored yet this year), he'd have just ~3 goals and ~6 points (Robertson has 7 goals and 11 points). Conversely, if Robertson had played as little as Reaves, he'd still have ~5 goals and ~8 points (Reaves has 0 goals and 2 points).
In short, despite playing very little, and not with the highest quality linemates, Robertson is one of the league's most efficient goal-scorers. Even this season, he's scored at a better rate at 5v5 than all but four players on the Leafs (Knies, Nylander, Matthews, and McMann), and last season trailed only Auston Matthews. In contrast, Reaves is one of the worst offensive players in the NHL, and he's poor enough defensively that whenever Berube plays him on the fourth line, once the game gets close and he needs to defend, he sits Reaves and starts double shifting other RWs.
So no, Reaves would not come anywhere near to matching Robertson's production if their roles were reversed, and neither would a good deal of NHL players.
I'm not saying Robertson is some elite talent. He's got a lot of warts in his game, and I'm not even sure he belongs in the Leafs' playoff lineup. But for a lineup that can't get any scoring outside its top-six forwards, sitting Robertson for a guy with zero goals this season would be stupid. Reaves is a sub-NHL talent at this point in his career, and even Berube clearly has little use for him.