There have been countless articles on draft pick value and the benefits of trading down at the draft versus trading up (outside the very early picks of the first round, which have disproportionately high value). At this point, it's fairly well established that trading down for more draft picks is typically more efficient, as the marginal value of a better draft pick decreases as you exit the top 10 or so. By trading down, a team is trading off some quality in their draft pick for much more quantity in the amount of prospects they can pick. This has all been documented very thoroughly by people like Rhys Jessop, Garret Hohl, Travis Yost, and many, many others. So I'm not going to spend much time discussing it.
What's interesting to me is how this idea relates to the Leafs. Notably, the Leafs traded down a couple times in the 2015 draft, something most pundits and fans deemed to be a clever move for a team lacking prospect depth. However, I've heard it said that with the Leafs prospect system having been rebuilt and improved, and with the army of picks controlled by the team, the time is right for them to trade up and add another blue-chip prospect to their collection. While I'm not against trading up in the right circumstances, I believe this idea is flawed, and that trading down is still the more efficient route for the Leafs, for the following reasons:
1. Prospect depth is fleeting
While it's tempting to say that the Leafs have an excellent prospect pool and that alone justifies going for quality over quantity, that ignores that prospect depth changes very quickly. This is because prospects don't stay prospects for very long. They graduate to the NHL, get shipped off in trades, or simply don't perform at a high enough level to justify keeping them. As they age, they move away from being prospects, and they move out of a team's prospect development system.
We can use the current Leafs prospect pool as an example. The Leafs have a lot of prospect depth, but a lot of the players in the development pipeline now will be out of it in a year or two. Of the forwards alone, there's a legitimate case for eight of them to graduate to the NHL full-time (Nylander, Marner, Matthews, Brown, Soshnikov, Hyman, Leipsic, Leivo). Not all of them will, of course. Some will stay with the Marlies, some will get traded, and so on. But a year or two from now, they won't be part of the development pipeline, and the last five will be on the older end of what you can consider a prospect. That means there has to be another group of players coming in behind them (and in a couple years, another group of players coming behind THEM). Continually trading down, even when you have a good prospect pool, means that you can bolster your prospect depth to withstand the natural churn of the attrition of young players.
2. Drafted players take a while
Another important consideration is the amount of time it takes a typical prospect to actually enter a team's development system, as opposed to staying with their pre-draft organization. For non-elite prospects, this typically takes an additional one or two years after the draft. This means that the players a team drafts in year X don't actually start taking up room in their development system until year X+1 or X+2. This mitigates the concern a prospect-heavy team may have that they'll run out of room and resources (playing time and roster spots) to develop these prospects; you're not drafting to replenish your system now, you're drafting to replenish it one or two years from now, at which point the aforementioned prospect churn has taken hold. As it relates to the Leafs, it means they don't really have to worry about introducing anyone they draft right now to their crowded AHL team. So there's no reason not to continue to trade down and accumulate a high quantity of draft and prospect assets. Which leads to my third point:
3. Don't name the piggies
Prospects are a crapshoot. The Leafs have some really solid forward prospects right now, but aside from the big three (Nylander, Marner, Matthews), none are a lock to do anything at the NHL level. That's not a knock on those players, it's a reality of prospects. Most don't make it. As a team, that means you always need to have a contingency plan - in this case, MOAR PROSPECTS. The reason trading down is typically efficient is because you get more bites at the apple (and don't pay a huge cost in terms of the quality of bites - perhaps this was a poor metaphor).
Beyond the uncertainty of prospects, having more of them means you can operate from a position of strength in dealing them. And make no mistake, the Leafs will trade some prospects. At some point, the Leafs will want to acquire a current, good NHLer. They'll have to pay up to do so (Vancouver excepted). Having more prospect depth means you have more chips to play, and a higher chance that the team you're interested in trading with likes one of them enough to want them in a trade. There's no reason to stop trying to trade down and acquire further prospect depth just because you currently have a deep bench. They're still valuable assets, both to you and other teams.
To sum up
Trading down in the current NHL draft marketplace is efficient. As I said off the top, this has been established and re-established. For the reasons I detailed, I don't see the logic in going away from an efficient strategy, as it will continue to reap benefits, even for a team with solid prospect depth like the Leafs. There are certainly times where trading up is opportune. I've said numerous times that I would've been very happy to see the Leafs move up to select Kyle Connor last year, once he began to slip. But to me, those times are much fewer and far between than the times trading down is the better move, and that's true regardless of the health of your prospect system.