I do some work sometimes as a party entertainer in princess costumes, and I've been a Disney fan since I was small. The whole Disney princess marketing avenue is relatively recent, I think it's a result of everything for children becoming more commercialized and drawn down the gender lines.
The first three princess films (Snow White, Cinderella and Sleeping Beauty) were never made with the idea that they were going to sell tons of merchandise, the studio chose films that had a simple structure and the princesses themselves don't take up most of the running time, they were rotoscoped, which is expensive, and there's more time devoted to slapstick action with the mice, the dwarves and the fairies because they were just easier to animate. Cinderella was done on a tiny budget since the film just before it had tanked at the box office, it probably saved the studio from going under.
When they started making the Little Mermaid the studio was in trouble again and they tried something that had worked for them in the past, a simple fairytale with a happy ending, and it was majorly successful. And because it came out towards the end of the eighties when media aimed at children was pushing toys and merchandise heavily, they got in on that too. Beauty and the Beast followed suit, and Aladdin and so on, and now the films are almost second to the merchandise.
I do think Disney, and the princess line in particular, cops a lot of unfair criticism, they are basically doing what all the other production companies do. Other media aimed at little girls, the likes of the early My Little Pony, Strawberry Shortcake, Rainbow Brite etc. plugged a 'tea parties and cupcakes' sort of genre and there was precious little outside of that to broach a different view. Rainbow Brite had her moments and her theatrical film was surprisingly dark, but if you wanted to see a female character triumph over adversity a lot of the time you only had Disney. The gender-neutral shows often had just one or two token female characters (Smurfs, G.I Joe, Inspector Gadget etc.) and more recently Dreamworks has been pretty godawful about this, actually worse than Disney in some ways.
I do think the princess line is valuable in its own way, even though they are merchandised to hell and back. They've at least tried to have representation for little girls of colour and tomboys, and they're still trying with Moana and Elena of Avalor. Little girls are continually getting fobbed off everywhere else.