Disneyland Paris is a great Magic Kingdom park that caters to everybody so I’m unsure why trended against having more mature attractions in MK. I think Disney at times think 12 year old boys and girls can’t handle a bit more of a thrill. My sister rode Spiderman and ToT aged 12 and loved them.
There isn't anything wrong with the ride systems at MK...most of the changes have been cosmetic over structural. The problem is the theming has gone to more "children" focused IPs and theme.
If You Had Wings->Dreamflight->Buzz Lightyear...the omnimover system is the same, but went from a serious look at the history of flight, to a cartoony history of flight and is now based off a childrens IP. Arguably, the ride is more interactive, but the theme is now based on a childrens cartoon. Not even based on the Toy Story name (which has adult appeal), but on the Space Rangers, which is a childrens spinoff cartoon.
Pirates went fro fairly dark and viscous to lighthearted and tame. The Pirates no longer chase the women, they aren't in town to pillage and loot, they are looking for Mr Depp and his map. The whole feel of the ride has gone from all ages with a mature theme, to kiddy cartoony.
Circle vision->Timekeeper->Laugh Floor...Same as Buzz. From highminded "edutainment" that was interesting and engaging to all to an IP based on scary monsters that want to make you laugh.
Heck...Stitch is one of these stories as its a dumbed down AE that lost all the charm and specialness of the original.
MK, while keeping similar rides, has morphed from a family park to a kids park.