It's a strange turn of phrase but I interpret "mortgage [against] the future" as "taking assets from the future to spend in the present". The thing is: future assets are replenished
every year... draft picks are magic beans, for a playoff team they are hardly precious at all.
So on one hand, he rarely traded futures (1st round picks, 2nd round picks, etc). On the other hand: he pretty much never added high-value futures either, prospects were busting left and right, his AHL team was moved to Mordor (Newfoundland), he retained coaches who were not known for their ability to help young players thrive (Therrien and Lefebvre in particular), he didn't have a suitable development staff, he had a bare-bones analytics staff... that's all working against the future!
So never adding either young or peak players to the core was actually in effect 'mortgaging the future'. He retained futures in order to be able to use them at another time... a time that never came and the futures turned out to be bums because of bad scouting and development, in the end.
He had Carey Price and never added a suitable forward, or more specifically first line centreman, to the roster. He didn't INVEST in his team because he wanted to be able to so at a later date, a date that never came. He swapped but he couldn't add. He swapped 3OA Galchenyuk for Domi for Anderson, Subban for an older Weber, he swapped the young Sergachev (in a position of need, PMD) for Drouin, and he sold Pacioretty for assets. At no point did he straight-up add additional or excess talent to the team long-term... and at no-point did he actually sell multiple present assets to secure more futures.
So both squandering the present and also not selling assets for a rebuild... that's mortgaging against the future.