Why would anyone care how deep someone's speaking voice is?
Well, as time goes by human voices tend to deepen, and that of course, has an affect on your singing voice if you're a singer and you don't take care of your instrument
For you to not use gimmicks, you need to sit on your ass for 3 years, put out 3 singles that you abandon because the album's not good enough, make your fans wait even more to the point where most of them give up, put out the album, cancel performances, postpone / cancel tour dates and then finally do this:
Well, as time goes by human voices tend to deepen, and that of course, has an affect on your singing voice if you're a singer and you don't take care of your instrument
Britney's instrument is autotune.
Remember she uses it like Bob Dylan uses a guitar.