Lets look at whos leaving first, the Leafs lost Ilya Mikheyev, Colin Blackwell and Ondrej Kase to free agency and saw Jason Spezza transition from player to front office staff. On the ice, Mikheyev is the biggest loss here, but lets be honest, the 4 year, $19M deal Vancouver inked him to is a massive overpay. The Leafs are much better off finding the next Mikheyev than paying him that much. Losing Spezza hurts, but it hurts the heart much more than it hurts the team on the ice.
Of course, until the season officially starts, we won't know exactly what the Leafs bottom 6 looks like, but the replacements for the 4 who left look to be Calle Jarnkrok, Nicolas Aube-Kubel, Adam Gaudette and Zach Aston-Reese. Of course, Aston-Reese is on a PTO, so he isn't officially a Leaf *yet*. David Kampf and Pierre Engvall will be the only two returning members of the Leafs bottom six.
So, what's the difference? Both Aube-Kubel and Aston-Reese are known for being relentless, physical forecheckers who are also responsible defensively. Jarnkrok is another forward who is excellent defensively, who's career average for points per game is slightly lower than Mikheyev's 0.41 vs 0.49. The big difference here is that Jarnkrok doesn't spend nearly as much time on the injured list. Over the past three seasons he's been in the lineup for 33 more games than Mikheyev. As for Gaudette, he doesn't score at the rate Spezza did, but he throws double the hits, something the Leafs have lacked in the past.
Will the bottom six be better than last season? Its difficult to say, but its clear what Dubas is trying to do. He's building a more physical, defensively minded bottom 6 that can grind teams down while providing some offence in the process. He's also doing it on a shoestring budget, which allows them to spend as big as they do on their top 6 and their defence. It may be a bold prediction at this point, but I think Dubas has built a bottom 6 that will work better than last season's. Even if they are the same or slightly worse, the added physicality and intensity will make them a better playoff team, and lets be honest, at this point that's all Leafs fans care about.