I do agree that they are not nearly as hurting as some may believe they are.
I think the introduction of this dumbass SM thing - who I also believe is just a babysitter/SM girl they know and not a real legit I-actually-do-this-for-a-living SM professional - is just one of their newest lame attempts to seem "hip" or "with it" and, just like with most tit they try and throw at the wall to try and stick (e.g., the website no one goes to, the podcast they stopped doing), it'll probably just eventually fizzle out.
I will say, however, that I'd start to be a bit scared if I were them as to how they're going to sustain their lifestyle heading into their late 40s/early 50s....and beyond (similar to what you referenced re: transitioning)
Children are expensive. If they continue to see dwindling numbers (due to their audience moving on and/or evolving in their "likes") while still taking ridiculously expensive vacations or buying tit left and right, they are, indeed, going to start feeling the pinch even more.
If they choose to keep the kids off the vlogs (which I support 100%), then they're going to lose that segment of their audience who were only watching for the kids (e.g., the aunties and grandmas).
And they've probably already lost a good chunk of the theme park enthusiast viewers because they were showing the kids too bleeping much, and those people have already moved on and aren't coming back.
So they're left with stan freaks and passerby folks who happen to catch a vlog here or there, with a real sub base that's more like their livestream #s.
In short, their lack of focus in their vlogs (e.g., home vlogs, theme parks, kid-centric vlogs, etc) - while it may have worked when they were younger and/or in the short-term - will be their downfall when it comes to strong sustaining audience #s heading into their late 40s/early 50s.