Given that there is not a lot of sales data in the public, no one outside these companies actually knows how well DC is doing, it is just the vibes from fans are more negative around DC than it is around Marvel right now.
What DC characters have been replaced?
Right now anyone can still buy the original Batman, Wonder Woman, and Superman. Jon's Superman and Yara's Wondergirl (before it was cancelled due to continuing delays) were selling better that the original all the original characters except for Batman.
It's just weird that people will blame the existence of 4 to 5 'diverse' books for tanking the sales of a company that publishes over 40 books per month when half of the line is dominated by one character.
Alonso, most likely did what the company wanted because the fact that so many of these characters are transitioning to movies and tv shows so quickly shows this is the direction the company wanted to go. Sometimes someone is let go because a company wants to present the image of change while continuing in the same direction. Since then Marvel has created diverse 'replacement' or legacies for Black Knight, Iron Fist, and Darkhawk they have gone back the Sam Cap, Jane Thor has a mini etc... this tells me that from the companies point of view they have benefited from this strategy it is probably the reason why their closest competitor is copying it.
While I agree that replacing so many characters at the same time did cause a backlash by fans but Marvel sales never tanked, just look at Marvel's market share during Alonso. The only year the industry had decreased sales was 2017 and Marvel was still dominating.
Unit Share
2011: Marvel 40.93% DC 36.77%
2012: Marvel 37.59% DC 36.75%
2013: Marvel 36.97% DC 33.35%
2014: Marvel 36.78% DC 32.47%
2015: Marvel 41.82% DC 27.35%
2016: Marvel 39.30% DC 33.71%
2017: Marvel 38.30% DC 33.93%
Dollar Share
2011: Marvel 37.29% DC 31.41%
2012: Marvel 34.06% DC 31.94%
2013: Marvel 33.50% DC 30.33%
2014: Marvel 34.38% DC 28.86%
2015: Marvel 38.74% DC 25.75%
2016: Marvel 37.44% DC 30.02%
2017: Marvel 36.36% DC 30.07%