You're not wrong here on declining ad revenue, it's definitely a factor in paywall moves like this one, but not the only factor. And as others have pointed out... in some cases it may be too late to save the "patient". There's a whole lot to this discussion that's difficult to cover in a short post.
The reality is that the global advertising duopoly that is Google and Facebook has lead to a dramatic decrease in ad revenue for media outlets. The scale and control these two juggernaughts exert is near impossible to compete with and in many cases independent media and small groups like Future (relative to the tech giants), are now earning cents in the rand for every 1000 subscribers / viewers / eyeballs vs. what they were able to earn 10 or so years ago.
As much as it pains me to acknowledge, Google and Facebook both offer easy-to-use ad platforms with rich targeting capabilities, DIY facilities and the ability to spend R100 or R100 million depending on your budget. Publishers have tried to compete with bespoke offerings, content partnerships and other exclusive opportunities with some success. But ultimately more ad spend continues to move to the big platforms. Moves like Apple's anti-tracking and regulation like GDPR has slowed their growth, but they'll find new ways to track and identify users.
It's easy to place the blame on the big tech monopolies and call it a day though. But in part, internet media and platforms are their own worst enemies when it comes to creating a culture of free. Only to later alienate users with increasingly invasive advertising and progressively weaker content. In that space it's a race to the bottom in both quality of the content / offering and the ad revenue that can be derived from it.
The business model of purely ad-supported content is broken. And personally I don't think it should be or can be fixed. The industry is in the midst of a big, much-needed reset.
In think that in time economics will prevail, but that's not going to happen overnight.
In an ideal world, if you as a reader / member see sufficient value in a publication or platform you should be willing to pay a reasonable fee for access to it. If you and enough others don't see sufficient value then perhaps it wasn't worth producing in the first place (economically speaking). Those that are low-quality and don't deliver value will falter in time.
In the cycling media space there are some stand outs like GCN+ and CyclingTips as some have highlighted here who produce great content and have a subs fee deemed "worth it". But then even a platform as abundantly pervasive as Strava is struggling to convert free users to paid subscribers. There's some work to be done by many.
In the case of CyclingNews, I see some chirps about the capitalistic, money-driven owners. While I'm sure shareholders and quarterly performance has driven many decisions, I'd hazard a guess that this is more of a last-ditch (and likely too late based on comments here) effort to stay viable rather than some super-car driving mogul looking to fund their next play toy.