The Leafs are in a bind with this core.
1. Over 82gp regular season games the Leafs core is likely the most entertaining and exciting in the NHL. They are fun players to watch, win a lot of games, and make the Leafs relevant. They seduce you with their talent and continually give you hope that this is finally the year they put it all together and make a Cup run.
2. If you breakup the core and ice a team that is less exciting during the season, there's still no guarantees you win a Cup. So you're sacrificing 82 games worth of high-tempo excitement for a new hope of winning. This could easily backfire.
3. The Leafs cannot afford to make a mistake in any trade involving the core 4. They absolutely must make the team immediately better without losing anything in the process. This means we are at the mercy of other teams willing to offer a fair return. Kids like Matthew Tkachuk likely only get traded once -- we missed the boat with him. Do we truly want to deal a Marner, Matthews, or Nylander for a bucket full of multiple "meh" pieces? I don't. Unless these kids price themselves off the team, then the Leafs are completely screwed.
The Leafs brass realize only one team out of 32 get to win a Cup each year, so the odds are heavily stacked against them. But they realize they have an uber exciting team that keeps the fans excited and engaged in the product full-time. They don't want to mess that up. They also have invested a ton in this core and hope to see their investments pay off.