I think if the Leafs swapped out MM, JT, and Rielly for Bennett, Marchand, and Ekblad, they'd be heavy favorites to make it to the Cup Final. This is even if the Panthers sign someone like Boeser with their $19M or so capspace heading into next season.
Now here's a hot take....I'd rather re-sign MM, JT, and keep Rielly and run it back knowing that the Panthers will lose 2 or all 3 of these 3 free agents. The Leafs took the Panthers to 7, and I don't see the Panthers beating the Leafs again if they're missing these core players.
Signing Bennett, Marchand, and Ekblad will help you in Year 1 but have significant diminishing returns. Ekblad is going to be 30 and is a complete force, but his physical age is probably closer to 36 or 37 given his mileage, previous injuries, and PED use (to cover up chronic injuries?). Marchand is literally going to be 38. Bennett would be a solid addition, but not at $10M X 7.
I'd actually rather take Marner on $13.5M X 8; Tavares on $5M x 3; and Rielly as with a $3.5M AAV buyout if needed because it's less a hell lot less long-term risk than signing those Panthers to long-term deals.