They are not just introducing a fee for the first bag. They're also increasing fees for the second, third, and fourth bags, increasing the fees for overweight and oversize luggage, increasing the fees for a checked kennel, and on top of it, increasing the fee to upgrade to Plus at check-in for longer flights. This is one big cash grab.
And it's coming from one of the most profitable airlines in North America -- one that consistently turns a profit, and exceeded it's return on invested capital goal (12% -- already high for an airline) for the last two years. They don't need the money. This is just corporate greed.
WestJet was built on the premise of offering high-value customer-friendly service. This has slowly been whittled away over the last 5 years or so. The lack of baggage fees was one of the few things left. Now, WestJet has grown up and is just the same as everyone else -- except they still have an awful frequent flier program.