Every game company with a recurring subscription wants to increase their total number of subscribers. Slowly phasing out prepaid time is a poor guess at Blizzard’s strategy, that seems very unlikely. The cost of prepaid time was increased in Canada because that country is implementing a digital goods tax. Blizzard chose to pass that tax on to consumers and it seems likely that other digital goods providers, like Amazon and Netflix, will do the same. See my post here.
I can’t see that they have “upped the price for all other regions,” I only see announcements AU and CA. AU was paying $11USD/mo and their price hike brings them in line with US and EU at $15USD/mo. Please cite other regional price increases with some source.
Canada’s price hike has to do with taxes Canada implemented–nothing to do with inflation or exchange rates (see response to Bae above).
This is really all you needed to say about a potential US price increase. The implications about other regions having price increases are irrelevant. According to the US CPI Inflation Calculator, $14.99 back in Nov-2004 has the same purchasing power as $20.42 in Nov-2020, so US inflation over 16 years is adequate justification for a US price hike to $20/mo; no other reasons needed.
They would be foolish, imo, to implement a US price hike now rather than wait until the US economy rebounds after Covid inoculations are widely administered. I wouldn’t be surprised if they did something foolish though–look at recent Torghast changes! 
…or the recent price hikes to AU and CA which also could have waited for economic recovery to some “new-normal.”