I don't think it's fair to point a finger at WEM for any negative effects on downtown. Downtown has its own issues which are predominantly unrelated to any competitive effect from WEM, such as homelessness/drug use/aggressive panhandling, parking rules and enforcement, the ongoing lack of rapid transit access from key areas of the city, a public perception of lack of safety on the LRT routes that DO run to and from downtown, and an unwillingness or inability by landlords to adjust rents in order to attract and retain unique businesses.
You could blame WEM if key tenants were leaving downtown and relocating to WEM or consolidating in their existing locations there. They aren't. Holt Renfrew didn't move to WEM, it left Edmonton entirely. Hudson's Bay exited ECC and consolidated its central Edmonton operations at Kingsway. Sport Chek announced that it was leaving ECC not because of the competitive effects from its store in WEM but rather because it considers downtown an unsafe and unattractive environment which is deterring customers.