... We don't know if Titania has been replaced.
We do, though -- Mab & Titania were born twins. Presumably they became fae around the same time, with a pair of Ladymantles.
We have "over a thousand years" that Mab has been Mab (i.e. Queen of Winter) and I suspect, particular, it was about 1330 years ago (roughly Arthurian era) -- because that's 2x the 666year Starborn/Outsider cycles (rounded down to the nearest decade). Those big Outsider-Wars are hard on the Fae. Or maybe go for 3 of those cycles, and we're back to the time of Yeshua son of Mary ...
We don't know how long Mab & Titania were Ladies, though. Immortals, they could have lasted a long time!
Even the names aren't great clues -- "Mab" is Celtic(Brythonic) in origin, "Titania" is Latin, first recorded in Ovid.
Maybe a Celt/Roman couple who had twins, and named one for each background?
But in any case, the Queens have been replaced much more than the Mothers have;
and the Ladies more-often still;
and the Knights get used up like a box of Kleenex during cold-season.