I don't think it's that simple.
There's no denying WEM had an impact on Downtown retail/entertainment. So did Kingsway, the LRT construction in the '80s, and later big box stores and online retail. What happened with WEM is what happened with a lot of major cities -- the regional shopping hub moved from city to suburb -- and while other cities have clawed back at malls in recent decades, in Edmonton, WEM is still the biggest retail experience and of course the greatest draw. We see this today with most new international retailers to Edmonton setting up shop in WEM first and occasionally Windermere, Southgate, and South Common. Even when they expand, they usually don't come Downtown. If they want an urban format store, they tend to go with Whyte, though even it pales in destination (chain) retail compared to Calgary's 17th, let alone Robson, Granville, or West 4th in Vancouver.
But not every retailer only has one or two stores in a major centre. Many are well-entrenched, especially national brands, like the Bay or Sport Chek. I don't think these retailers (or Holt Renfrew) left City Centre because of WEM. They departed well after the mall had established retail dominance for the metro area and have plenty of locations throughout Edmonton and were viable in the core for decades. This isn't the 1930s when Edmonton could only support 1 Bay store. So in some cases, the blame can be put squarely on the state of Downtown Edmonton, rather than the oft-repeated claim that malls are cannibalizing inner city retail options. Like, sure, maybe it's why LL Bean or Yves Saint Laurent aren't Downtown, but the Bay and Sport Chek?
A good case study for Downtown Edmonton retail might actually be Detroit. Despite the stereotype, it's done a remarkable job at revitalizing the Downtown and a few key central neighbourhoods, and has better retail options than ours. Which, maybe you'd expect, as its the core of a 4 million+ metro, but it wasn't like this until fairly recently. Gucci, Nike, Warby Parker, Lululemon, Bonobos, H&M, alongside more local options are all in/around Woodward Ave in Downtown Detroit.
This is what they're doing on the site of an old downtown department store. If they can turn it around, so can we.