Nah, Hasbro are the dicks. Unfortunatly, the writers are restricted by their bosses who are more worried about selling crummy toys (like Bratz rip-offs-*cough* Equestrial Girls *cough*) than making fans of the show happy.
every author is being called a dick by the fanbase, if he does something "terrible" with a beloved character.
andrew hussie (the author of homestuck) is a good example for this case. he let many popular characters die (he even let himself die), everyone begin to hate him and he laughs about it. you don´t need to read homestuck to find this out, you just need to google his name and you get every information you need.
Hasbro's always done that. I grew up in the 80s during the Golden Age of G.I. Joe. There were always re-colored versions of characters. I remember at least 3 Flints, 2 Sgt. Slaughters, 3 Wild Bills, etc. They had the same body mold but all colored differently.
Rarity did get jealous. In the chapter book celled "Rarity and the Curious Case of Charity" Spike develop a crash on Rarity's appreciate Charity Sweetmint and Rarity start to shows signs of jealously. Especially when Charity addresses him as "Spikey-Wikey".
Except that pony starts replacing Rarity in every way and she even 'steals' her 'Spikey-Wikey' routine. Big difference between smeone taking someone's place and someone giving a friend positive attention.