I am a player that plays at my own speed and for my own enjoyment but I don't see the point in upgrading temples and wasting food just so I can progress through the Adventure Map.
If anything, the most enjoyment I get out of the game is earning food/gems/cells in the Dungeons, and if I power up some monsters to a new max level, then all of my opponents in the Dungeons will be a higher level, too.
This means I will have to power up ALL of my monsters just so I can interchange them for fun in the Dungeons (rather than use the same 3 monsters all the time).
I think Level 70 is high enough for my monsters- and of my 25 legendaries, only a handful are that level anyway. Food is too scarce in the game to power up EVERY monster to level 100 surely?