I think Stella in her prime was certainly deserving of principal at a top tier company. She still has so many lovely qualities as a dancer, but IMO her technique is not as strong anymore. All those years when she was dancing Lilac Fairy, Myrtha and Gamzatti and holding down the fort as a soloist she could have been a beautiful Aurora, O/O, Giselle, Nikya, etc. Alas now it's too late. Don't get me wrong she's a wonderful dancer still but her career never developed the way it should have.
As for Lane I believe she also is in her mid-30's when she finally got promoted. Again, we'll never know how her career might have progressed had she not spent so many years dancing the peasant pas de deux.