Most names date. So will Ottilie. So will pretty much every other name that is used more heavily now. Because people age. And when people are older others find their names less attractive because they grew tired from hearing them all of the time or associate them with older people. This is normal and okay. I don't really understand the point you are trying to make because no one is denying the fact that names date and it is completely normal and fine.
Names are used in 100 year cycles. So it is completely normal that Susan and Julie still sound tired and old because their comeback will be in about 40 years. They are just not ready yet. It's not their time yet.
Boys names are different because parents are way more conservative when naming boys. This has only changed much in the past 20 years or so. Andrew, John, Michael etc. were always used. However this is changing now too. When you look at the statistics you will see that these names are falling rapidly. Soon people will think that Andrew and Michael sound sort of old and boring. They are classics, they'll be back. But hardly any names is always fashionable. That is the point.
The only names that don't come back in cycles are usually the modern, new names that have never been used before and have no long etymology (like Jayleigh or Deneen). Unless they have literary associations that will appeal to future generations (I could see it with Lyra). Ottilie is not modern so it will come back in cycles just like it is doing now (it was first used more heavily in Britain in the late 1800s/early 1900s).
When you think about it a truly timeless name is hard to find. Elizabeth is one but that is because the nicknames that were used dated and not the full name itself. So Betty sounds 30s, Liz sounds 60s, Lizzie sounds 80s and Libby 90s. And boys names are a different story because of what I mentioned above.
Even Oliver was really unfashionable between about 1900 and 1970. Rachel, Rebecca, Sarah are Biblical classics but are unfashionable now. Deborah is a Biblical classic but unfashionable now. So is Judith. It's okay, they'll be back and are still fine names.
Ottilie won't date any worse. It is not Deneen or Jayleen or Jakaylah. It will date along the lines of Ava, Cecilia, Clara, Ada etc. and that is fine.