I went into this looking for it to be subversive, in part because Dumas was writing during the lead up to the revolutions of 1848, where I would have expected his sympathies to be on the side of the insurgents, especially given that his adored father was an ardent Republican who was badly treated by Napoleon in the long run, although he served him brilliantly in the field. Then, very early on, I thought Dumas signaled very heavily what we were to make of d'Artagnan, when after his father gave him his horse in trust, committing him to treating the horse well and giving it a comfortable retirement, d'Artagnan no sooner got to Paris then he sold them. But perhaps that was the message; that no father can expect entire fealty from his son, who has to find his own way.
Ordinarily I'm no fan of discussing a text in terms of a writer's life, but in a case like this of popular literature, I don't see why not.
As I think everyone agrees, I also find the characters to be inconsistent and the message, such as it was, also inconsistent. I found the scene where d'Artagnan returned to Aramis to find him on the verge of holy orders and the exposition of religious philosophy to be subversive and hilarious. Then, of course, Aramis while Aramis throws over religion, again, on receipt of a missive from his lady love, he returns to it eventually, and seemingly wholeheartedly. Perhaps a bit of St. Augustine's, "Lord, make me holy, but not yet," going on there. Although I find no inherent inconsistency in his postponing holy orders for a stint as a musketeer in a time of religious wars.