"There's always been a market for 'lifestyles of the rich and famous' genre."
Yes, there is. But with a nuanced and important difference. The 'great unwashed' and 'deplorables' DO enjoy watching rich people being rich...as long as those programmes include some kind of drama or unpleasantness with which the rich people must contend.
The programmes you mention are popular because they portray rich people as messy, dramatic, and ultimately lead the viewer to the comfortable conclusion that money isn't everything. These type of shows are only successful as long as they promote some level of comforting schadenfreude in the viewer. On a psychological level, we could posit that they allow the viewer to make peace with the ridiculous wealth inequalities in the US by suggesting to them that being rich isn't all that great if you also have to deal with family drama, bitchery, bad plastic surgery and fakery. Thus the viewer can take comfort from concluding that their own life, humble though it is, is in some measure more satisfying than the lives of those they are watching on TV.
ALL successful 'lifestyles of the rich & famous' scripted reality shows offer this vital element of schadenfreude.
With Love, Meghan will not do this because Meghan can't let it. Ever since Megxit she has sought to tread a very flimsy and confused line of presenting herself and Harry as spiritually enlightened philanthropists who have been through the fire of suffering and have reached a new plane of serenity and wisdom. But on some level she knows this doesn't quite hit the sweet spot, so she's consistently tried to mix in some element of comedy (fake curtseying, being squirted in the eye with lemon juice) so she can show that she and Harry are still relatable, y'all! BUT she could never let her self-presentation go to the level of Real Housewives and show cat fights, bitching and betrayal. Because if she did this, the viewer might reasonably conclude that Megxit was a bad idea, and that H&M's lives in Cali are empty and unfulfilling. And they could NEVER give anyone grounds for thinking this, because if they do, then what the hell was it all for?
TLDR: With Love, Meghan will be missing the crucial ingredient (schadenfreude) that makes other scripted reality shows about rich people, successful. Therefore, With Love, Meghan will not be successful.