Grabbed the headline from Reddit. If you read below the person who compared the articles did not realize when posting that the 2017 number was from PC online (2017 article: https://mmos.com/news/top-free-play-pc-games-revenue-2017-superdataresearch )
I am quoting someone who breaks it down better that I have time to:
Superdata is very tricky to interpret, and they screw up the analysis themselves sometimes based on the soundbites they issue. A lot of the confusion seems to stem from whether mobile sales are included with PC sales.
This is exemplified by the fact that every time the Hearthstone superdata is released with a PC report, usually around March, you’ll see doomsday warnings that Hearthstone year over years sales for the prior year has declined by 50% . See for example:
These reports are usually in direct contrast to reports which are released later in the year that suggests Hearthstone revenue has in fact grown since previous years. See for example:
We know from previous superdata reports that Hearthstone revenue is about evenly split between mobile and PC: https://www.polygon.com/2015/8/11/9130779/superdata-hearthstone-pulls-in-20-million-a-month-as-it-disrupts-the
Note that all of the reports I’ve cited use Superdata as a source, so it isn’t as if I’m cherry picking different sampling techniques from report to report. The most straightforward explanation to reconcile the early data of devastating decline in revenue in the early reports and steady growth reported several months later is that only PC sales are reported early, whereas PC and mobile combined sales figures are reported later. This would imply that the consistent 50% declines is just people doing crappy analysis by comparing PC-only sales to PC and mobile combined sales from prior years.
I am agnostic regarding whether this current report has made the same error as the previous early reports, seeing as it was published at nearly the end of May. Regardless, I take Superdata analysis regarding Hearthstone with a large dose of skepticism.
EDIT: thanks for the gold!
EDIT 2: Thanks for the second gold and silver! 
Like I said in my introductory paragraph, Superdata is tricky to interpret. So there are some additional corrections and discussions worth while thanks to in-depth review by u/saingshing:
- The $414M reported in the Aug 2 VentureBeat article does not refer to total Hearthstone earnings from 2017. Rather it appears to be the total North American/US Digital CCG revenue, as seen in this image. Notably, this figure would exclude other markets like Europe, APAC or China. Consequently we don’t have an exact figure regarding how much Hearthstone earned in 2017; it can be less than $414M, but I suspect that it would actually be more. My understanding is that 2017 was when Hearthstone revenue peaked, at least as of 2017, as supported by articles such as this. Therefore, my main claim that Hearthstone experienced steady growth from 2014 to 2017 remains mostly unaffected. If someone wants to wade through the previous 10-K forms to see if they provide any insight, they are welcome to do so.
- The Jan 30 2018 MMOs article does not contain any Doomsday predictions, nor make any direct comparison with 2017 earnings; that was my editorialization for clarity. However, it does fit the pattern as seen in the March 28 2017 and March 21 2019 reports, and provides insight in how a 50% drop in Hearthstone revenue could have been mistakenly arose.
- Notwithstanding the caveats noted above, I maintain that there has been irresponsible reporting about the impending death of Hearthstone, at least in 2017. We don’t have the 2018 revenue earnings to make any judgments regarding the 2018 reports however.
Since Act/Blizz doesn’t need to breakout HS’s revenue we don’t know the exact numbers but this is what I am looking at:
2016: $394.6 million ( source: Super Data)
2017: $414 million ( source: Super Data)
2018: If we assume the same 50/50ish split it appears 2018 is ~$370m using Super Data’s mobile sales data; however, I cannot find an overall sales data number for 2018 from anyone
2019: unknown obviously as revenue data has not been reported by Blizzard to begin forming good assumptions on Q4 sales
Facts are back in your court I guess… since reading more than a headline is usually a good thing ./shrug