Isla and Keira don't have the long history that Isabel/Isabella has, I think that they are in a completely different category. Even so, although they may not fare quite so well I don't think even they would ever get to S&T levels of farce in the '80s. No, if anything, it will be the name of the offspring of a 'celebrity' with questionable morals: sexually, financially, ethically...
I found some statistics on name popularity that reflect that some names have stood the test of time while others have faded.
For anyone interested, here are the top ten names for one year in each decade (I can't find the whole decade average with the most popular first.
1954 : Susan, Linda, Christine, Margaret, Janet, Patricia, Carol, Elizabeth, Mary, Anne
1964 : Susan, Julie, Karen, Jacqueline, Deborah, Tracey, Jane, Helen, Diane, Sharon
1974 : Sarah, Claire, Nicola, Emma, Lisa, Joanne, Michelle, Helen, Samantha, Karen
1984: Sarah, Laura, Gemma, Emma, Rebecca, Claire, Victoria, Samantha, Rachel, Amy
1994: Rebecca, Lauren, Jessica, Charlotte, Hannah, Sophie, Amy, Emily, Laura, Emma
2003: Emily, Ellie, Chloe, Jessica, Sophie, Megan, Lucy, Olivia, Charlotte, Hannha
Source: Oxford Dictionary of First Names (not my favourite book but it will suffice)
Sorry if your name is one of the ones I mentioned, whyno