There's nothing terrible about having a name that sounds dated.
But it sort of defeats the purpose, in a way. People choose names they really, really like, which sometimes happen to be names that other people really, really like - hence their popularity.
Those names are chosen because they sound so lovely to our ears right now. But the switch comes that through over-use they cease to sound lovely and instead end up sounding dull, boring and predictable, and what was once considered so lovely about them is lost.
I mean - how many Joannes, Karens, Jennifers and yes, Sarahs does our generation know? Loads. They no longer sound 'lovely' in the way they must have done when our parents were choosing them. They sound (no offence intended) boring, eyes-glaze-over, 'oh, another one'...
If you chose a name less likely to date, then the chances are that names goes on sounding fresh well into adulthood and old age. The thing lots of people seem to miss is that this doesn't mean you're limited to the timeless classics of Elizabeth, Catherine, etc.
There are loads and LOADS of names which won't date the owner to a particular generation!! If you simply avoid faddy names.
There's miles of middle ground between timeless & classic - and faddy. And this is why many people are so keen to avoid faddy.
Names from literature, slightly left-field but recognisable names. God, there's a whole world of names out there that will not pinpoint your child to a certain age group.
Off the top of my head, I'd avoid Ruby, Daisy, Lily, Molly, Tilly, Evie, Isobel/abelle/abella, etc, etc, etc.
And choose something a bit different. Rosalind, Miranda, Helena, Eleanor/Elinor, Mallory, Merryn, Celeste, Louisa, Elle, Honor, Hero, Fenella, etc, etc, etc.
Obviously not all these names will be to everyone's taste (they're not all to mine). But if you put your mind it to, it's ludicrously easy to choose a non-faddy, and also non-run-of-the-mill 'classic' name. So yeah, there's nothing wrong with a dated-sounding name per se; but when you consider all the names out there at our disposal to bestow on our child as one of their main defining features for the rest of their lives, you can see why some people don't want to choose the same old names as everyone else.