There are actually more reasons for the discontinuation of Traditional skins- I did a post with the reasons a few months ago, so I'll just straight-up copy it to here:
The reasons for not making Traditional Skins anymore were, as far as I remember:
- They were taking time to make- It's technically a new skin that would take time to design (Even if it's based off an already-existing design), and at the end of the day it would just 'delay' the update. (And no, Riot will probably never let us have the old MODELS for a Traditional skin, because, well, that's what they were looking to replace with the Update. And in my opinion, this is probably for the better, because the old designs are being replaced for a reason- they do not fit in with the game's modern art direction).
Also, the delay would not only come from designing the skin, but also playtesting it- It's basically a new skin, which usually takes at least two weeks (or probably even more) to playtest internally, give feedback on, make sure it doesn't have any bugs (Especially for Traditional skins which are usually feature-heavy, with at least "new" Voiceover and sometimes even new SFX, animation cycles and VFX).
- Nowdays, Riot don't do 'stand-alone VUs' or 'stand-alone big GUs' anymore, because it saves a lot of resources and is overall more efficient to either do full VGUs or Roster Updates. Sion was the first full VGU that really had a weight on the decision of making a Traditional skin- He got special lines for his new Ultimate, Unstoppable Onslaught, which are basically him screaming how Noxus is glorious, or the famous "COWWWWWWARDS!". His old Voiceover had none of that. Cases like these are also a reason not to go for a Traditional Skin. (Although, one could argue that Riot can put his old, silly laughing lines on repeat as his Ult's scream- Which I personally think would've been hilarious, but again, would've taken even more time to complete his update. Moving on...)
- They were skins based on old designs- Designs that Riot were looking to replace with the Visual Updates. Cases like Taric would've been weird if they got a Traditional skin- Riot built his new Classic skin off the old one, because they know there was some charm in it, despite being very old & dusty and unfitting for the game's modern art direction.
Also, Gangplank's Captain skin is not based off his previous classic skin (Prior the big update), and it was done mostly for lore & event reasons (Bilgewater: Burning Tides). It's not really a Traditional skin in the sense that it doesn't represent his older model.
(I hope this last paragraph was understandable- I mean, with my wording.)