Certainly sustained spells are convenient, but it makes for more passive play when you set up your buffs/summons in advance. Having to decide whether to spend my time/magicka to replenish a buff, summon a new creature, or cast a heal or an offensive spell increases the number of choices I make during a fight. It's understandable that a party-based game like Dragon Age needs to simplify character management a bit, but in TES we only play one character.
Ah, but in this system, you would still have to make those decisions since you have a trade-off for sustaining spells. Want to do more dps because the enemy type is low hp but high damage? Deactivate some buffs to free up regen/magicka so that you can cast more offensive magic. Battle is long and you need magicka to heal? Toggle off. Fighting spell caster monsters instead of melee? Deactivate armour and turn on spell absorb/reflect. You still have to make these decisions as you go along but now it's more convenient and you have an incentive to scout ahead so that you can prepare the appropriate buffs. Previously, there was little advantage to scouting since you'd cast the buff before the battle but usually it would run out halfway through (unless you had created your own long lasting spells with high magicka costs). With this system, you could assess the threat and prepare buffs appropriately.
To me, recasting does add a layer of complexity as you said, but it's not the type that I find fun. It would be like if they implemented a hunger/thirst system. Surely, you would need to think more about how far you're going to travel, how long it will take, whether you should leave a dungeon early to get out for food, maybe you would have to scavenge to find food, but none of this is actually fun (for me) to do. It's tedious and doesn't make me feel like an awesome hero (which is one of the whole points of video games - make me feel like I'm awesome).
Instead of babysitting my buff bar, I want to be thinking about who my enemy is, what they're weak against, direct damage or AoE?, positioning, equip weapon or all-out spellcasting?, play defensively (save magicka for healing) or all-in? I don't want to get into battle and keep glancing at my buff bar. The Elder Scrolls series consists of real time games without a true pause system like Baldur's Gate or Dragon Age. It is an action RPG and, as such, should have a focus on keeping the pace up and not being bogged down by watching timers up in the top corner of your screen while all of the interest effects and action happen in the middle.
Even when action (ie. combat) isn't happening, why do I need my exploration, that is, my admiration and inspection of the often beautiful handcrafted landscape put together by the devs, to be interrupted by constantly checking to see when my Light/Feather/Speed spells will wear off?
To me, all of these inconvenience take away from the game rather than add to it.