Well if Wikipedia says it it must be true! Nevermind the fact that the link in question in no way shows that the company that collected the data have suggested it is discredited.
As to your other "relevant links", I've already shown the flaws in their logic elsewhere. Not to mention, yet again, one of their primary complaints was that it was too old....and yet now 10+ years later the numbers remain largely the same.
If I ask for a source I'd put dollars to donuts you're going to quote articles from 2014. Those numbers work great for people with an agenda and who either don't know, or realize their readers may be ignorant of, the realities involved.
One of the largest factors in merchandising sales for teams are Jersey's, typically of star plays. In 2012, the Washington Redskins traded multiple picks and drafted a Heisman winning quarterback by the name of Robert Griffin III with the #2 pick in the draft. He would go on to have the #1 selling jersey in the NFL that year. Indeed, he would have the #1 selling single fiscal year in
league history. This has to major impacts as it relates to merchandising totals. First, to causes it to spike, as there is a MASSIVE influx of new purchases. Second, because of how many of his jersey's were sold in that single year, it created a large saturation of individuals with said Jersey.
Now, here's the flip side to that rocketing surge. The Redskins traded away their 1st round picks in 2013 and 2014 in order to obtain Robert Griffin. On top of that, they were hit with a massive salary cap penalty which kept them from significantly going out and being able to purchase high priced free agents. So the team lacked new "stars" in 2013 and most of the 2014 off season for fans to flow out to stores and purchase jersey's of. This is going to cause a natural regression in the merchandising totals, especially when comparing it to the artificially high set from 2012.
Combine this fan disgust at Snyder for quite some time here in Washington that has NOTHING to do with the name. Note, this was a fanbase that once created a
"fan card" mail-in campaign as a means of trying to send a message to the owner that they were unhappy with how he was meddling with things. And the 2013 into 2014 season, the sports talk in the DC area was against largely around the notion of Dan Snyder meddling in operations again, this time relating to RG3.
Now, is it possible that there was some damage done by the VERY small minority of the team fans (Because those are what would matter here; people who aren't fans of the team wouldn't randomly be buying redskins merchandise if not for this) that actually have an issue with the name? Sure. Is it likely that it was the primary reason, or even a significant reason, for a fourty percent decrease? HIGHLY unlikely and based on nothing but hopeful wishing of those who are ignorantly still clinging to this crusade.