They aren’t gonna have much of a team soon. When you look at all the recent stanley cup winners they have their best players signed to great deals. The moment they overpaid them (Kane,Toews for example) is when the window closes because they don’t have the money for good depth anymore.
So you think losing Jake Gardiner and idk, Connor Brown (?) makes this team so much worse they can't be a good team next year? Have to say i disagree. Leafs have pretty decent young players coming up to replace guys who, in this Boston series, were secondary pieces at best.
They're going to bring back 90%+ same team they had this year for next year, and after that Marleau's contract comes off the books and the cap goes up again, freeing up good amount of space to manage their cap and at that point replacing Marleau with a young guy will probably make the team better.
And as far as your Chicago example goes, the Hawks didn't become a bad team the moment they handed out those contracts. They had over 100 pts b2b seasons. Besides, one could argue the depth wasn't their biggest issue, rather the core guys declining. Seabrook turned from a minute eater to bottom pairing dman on a good team. Toews saw a decline in offense for years. Keith has been slowing down gradually. Marian Hossa was in his late 30's and ultimately 'retired'.
Comparing the Hawks aging core to the Leafs young core is laughable and they are not similar in any way.
To answer the OP's question, has the Leafs rebuild failed, of course not lol. As far as i remember they were a bottom dweller for years. Now they've been a playoff team for 3 years in a row, and have that young core for years to come. Seems to me like they went from a terrible team with no winning culture to a team that's seen as a great team around the league = rebuild successful.