In short- you can still use utility -based old chars for buffing\debuffing, but current balance killed 90% of old DPS chars (they`re simply outscaled now by rare\ modern quest chars).
Last 50 chars (dunno if that`s exact, but ~~ from the time of Codename L and SFLynn release) is the most destructive thing for the game balance ever. Pre-CNL there was a good reason to play 80% of chars, they was needed due to contexts and situations. Damage gap between stock&early quest chars and "modern" quest chars was not that big, and rare chars was usually pure-PVP based with defensive stats in their buffs instead of offensive in quest char` buffs. Compare SFL & Lynn, EJudith and Judith, Olivia& Spy Olivia- usually quest versions was more into damage, but rare char was able to survive better.
Since then...
CNL was the 1st char that was overall destructive for game meta. Her damage was too much, and job skill made her a good universal pick (i think until Cr.Daria release, that`s a couple of years!)
Not only Leona required way more stuff to use her (CTB weekly grind for book), but she was simply put into buffer role.
Next big destructive call was Aclla - she simply outdamaged every single MDPS on release. Her 20k% summary channelings was out of meta by any means, she basically 1-2shot most of actual missions. And then its began- Aclla, QSJoshua, Ruby...
IMC caputred themselves in their greed. They kept producing more and more powerful characters to milk money.
At the moment there`re only a few viable quest DPS, and their viability is mostly "well, it`s not THAT bad".
Dont get it wrong, tho. Most of characters, if equpped good & combined with some knowledge, can compplete almost all current single- player content (scenario, private missions, stuff). But it`s VERY hard to compete with rare (meta) chars (some of modern rare chars is still "for fun", not so focused on damage, but using interesting mechanics, like Hildegarde, Nike,Freya,etc. They`re out of this, ofc.) using even best quest DPS.
So...you can pick someone like Calyce, go till the lategame with her solo, but in lategame she`s simply outscaled by Moon\MSB. Same with other DPS categories.
Tho that`s where Andromeda have its biggest chance to shine, as you`re able to get even chars which is P2W on other servers. I see it as the top important feature, tbh.