I feel like Kessel and Marner are comparably talented players (relative to how they rank league-wide), but Kessel was a better play driver while Marner is a substantially better all-around player. That's why I think the 2015 Kessel trade is a good estimate for what Marner would bring back in a trade, I think it would be like 2 1sts (or a 1st and a 1st round prospect), another lesser prospect and some cap going back.
Like I said above, I think Smith, Pickering, Ilyin and a lottery protected 1st for Marner is really in line with the Kessel trade and is pretty realistic for what a Marner trade would end up looking like. Smith is an interesting guy for the Leafs to target due to his superb playoff track record, they can likely just slide him into Marner's role and see how Smith would do in the playoffs for them.
The problem isn't the "value" -- it's that that type of trade doesn't really do anything for the Leafs. When the Leafs traded Kessel, they were about to embark upon a complete rebuild. The Leafs trading Marner is a retool move.
The future pieces don't really serve much of an organizational need, nor do the Leafs have a good way to develop them, and Rielly Smith, more of just a "placeholder" to throw onto the right side.
Obviously the Pens don't have a ton of guys that really fit well with the Leafs... but none of those assets really move the needle.
A guy like O'Connor -- might flame out into nothing, or might be a really important piece alongside Matthew Knies, Bobby McMann and maybe Max Domi. A guy like Brayden Yager, might be able to compete with Fraser Minten for a spot in the lineup next year, and give the Leafs a guy that can grow as Tavares declines.