Opposition for the Sake of Opposition
However, this smells a lot like a party opposing something for the sake of opposing it. Because the reality is, there's no way the Liberal Party could ever follow through on this pledge if they were in power.
Given the screams of horror that come out of Liberal ranks whenever the NDP even hints at opening up the constitution, it's a safe bet the Liberals aren't about to go down that road anytime soon. Which means each province must have at least as many MPs as it has Senators, and cannot have fewer seats than it had in 1985.
So even if the Liberals wanted to piss off a few smaller provinces to achieve true rep-by-pop, their hands are tied - Saskatchewan, Manitoba, Quebec, New Brunswick, Nova Scotia, PEI, Newfoundland, and the territories are all at their 1985 seat totals.
If the Liberals want to keep the total at 308 seats, all they can do is shuffle chairs around between Alberta, BC, and Ontario, unless they repeal the laws of mathematics. Come to think of it, that might actually be easier than opening up the constitution.
So while there's some value behind the intent of this position, it's completely unworkable in reality unless the Liberals want to leave Ontario, BC, and Alberta seriously under-represented. Which might be a good thing, given the number of votes a plan like that would cost the Liberals in those provinces.