I dont think you understand what the NHL is about.
The ultimate goal is to win a cup. I can guarantee Tampa(any team) will mortgage the long term future if they can win the cup. Every team mortgages the future to secure a cup victory. Why else do you see the price paid for players moved for picks/prospects.
Cap space is useless if you dont have the right players to use it on. If you think teams wouldnt pay 8mil for ppg playoff performer, you are dead wrong. Look at other players paid in that salary range.
Oh, trust me I understand what the NHL is about.
Of course the ultimate goal is the win the Stanley Cup but not every deal makes sense to win the cup. Again, of course you would pay Kessel 8M a year but it has to make sense for both teams. Let put together some hypothetical players that would be in a deal.
Players you will NOT be getting from Tampa Bay:
Stamkos
Hedman
Johnson
Bishop
Kucherov
Palat
Drouin
Players that would NOT make sense for the Leafs to get:
Callahan
Carle
Garrison
Coburn
Stralman
Filppula
Boyle
Ohlund
Up until this point that is 60M cap space which will not be moved. This doesn't even include extensions to players like Stamkos.
Now here are players that could be in play for a deal:
Killorn
Vasilevskiy
Brown
Namestnikov
Paquette
1st (NYR)
Deangelo
Erne
Koekkoek
Nesterov
Now with this list of players we could try and throw together some deals. Keep in mind Tampa will be approaching a max cap without including any of the possible players on the cap. Also keep in mind that having ELC and bridge contracts are key to success in the NHL.
We don't want to have Killorn, Brown, Paquette as center pieces of a deal. This shortens our list even more.
I mean sure if Tampa was willing to deal Erne + DeAngelo + 1st for Kessel then we could negotiate a deal. Does this deal make sense for the Leafs? I think so. Does it really make sense for the Lightning? Probably not. It will put themselves into cap problems in the future and they already have a young and growing team that came within 2 wins of a Stanley cup. Progression from other players within the organisation at a cheaper value will bring them more than Kessel.