Fire Lightning Ice aren't any more or less destructive than Floods Quakes/Volcanoes(which are fire/earth) and Tornados, actually of them Ice as an element is one of the least destructive on it's own, without the impact of wind to create a blizzard, earth to create an avalanche ... Ice actually creates very stable, if slick, homes/ground for people, fire is life giving in every corner of the world and every culture, lightning is what's making this whole conversation possible by being in a controlled form. Most poisons are derived from a liquid base.

I suspect a big reason they went with the current system was their need to have every ability from the current class/job on a persons action bars. Working with that limitation this way is perfectly fine AS LONG as they develop the content around it.
They haven't got nearly every ability and skill from the old/orig system. When they decided imposing a limit of 15 abilities to each class I have to believe it was to speed and simplify development for a version of the game that they knew was going away.

I really do hope that they don't keep this system, both on the 15 abilities front, and the way mages are now, whm isn't a whm from the majority of the old ff lore, blm isn't a blm from the majority of the old ff lore, yet the claim is, "We drew from previous FFs." What I mean by this is, to clarify, WHM hasn't every been a job that has a DD repertoire, BLM has always been the job that has elemental control to a science. This we have 2 Elemental mages not a whm and a blm, the only distinction is that blm can't heal and buff as well.
What I mean by Fire/Lightning/Ice being more destructive, is that they are, in and of themselves, destructive. While water is literally life giving, earth and wind are typically more serene, being that they are not exaggeration in and over themselves. Yes, anything can be come destructive, but honestly, if I REALLY wanted to I could probably kill someone with a rubber dildo. Dividing up the elements like it is has cool thematic possibilities.

I think with the 15 ability limit is so they have room to grow. Making sure that every (most) abilities are useful, and making sure they'll all be able to be within quick reach was a rather specific design choice. With the 15 from current class + 10 cross class abilities, they have room for 5 more when they raise the level cap. With jobs, the 15 class abilities + 5 cross class abilities + 5 Job abilities, does the same, it leaves 5 more available slots for when they raise the level cap.

Complex but easy to understand was achieved in many of them, with 8 elements the system isn't any more diffucult to understand just more robust, there's more options, more you CAN do, if someone still wants to cast nothing but the one spell they can, but those that want the more robust complex system are welcome to use it.
Just because there are more elements, doesn't mean it was more robust. It was a rather shallow system, being that there were no layers. To this day FFVI, when used properly, had a very layered magic system. Sometimes it was simple, other times it was hard to tell what element to use, when to use it, and how to use it. FFXI really was just matching up with the wheel.

They are! Just every bit as good as thunder, I'd say! In their particular roles. For diminished damaged, Fire gains AoE and it's just as good at AoE damage as Thunder is good in ST damage (Thunder-level damage to an AoE would be overpowered), Blizzard exchanges damage for enfeebling statuses and in Blizarra's case instant cast. Blizzard is as good at enfeebling as Thunder is good at dealing damage! They have very distinct roles and are equally proficient, in their own roles.
I think he means more of an in general stand point. When you run into a single strong earth enemy, you kinda get screwed. Sure each of the three elements have a situation where they really shine, but overall it mostly comes down to single target = Lightning, multiple targets, that won't all turn on me easily or die fast enough = fire, and ice well, it is what it is.

Presumably so cheap tactics like 'stunlocking' aren't usable and so that 'trench battles' become an uphill challange, making the playerbase shy away from excessively long, drawn-out battles and adding a more dynamic element to battle. Wether they implemented that correctly or failed at it, I cannot say, but it's possible to discern the intent of it.
I really do agree with him when it comes to enfeebles. I feel kinda sad without them. Though in order to fit them in the current system, they'd have to remove something. Which I think they could do if they also rebalanced existing abilities, and maybe also add some traits.