I hope SE is thinking this over carefully, because I just see this making a mage's job (Especially for Conjurers now.) ten times harder. Who's ever found the annoyance of having to spam Protect, Shell, Stoneskin, and Stygian/Shock Spikes because someone decides to run off or the whole party does? So you have to chase after the group just to buff them, sometimes getting all or most of them, other times maybe 3 including yourself. Personally I have never had any problems cancelling a spell, you hit the same action hotkey as it's casting and it'll stop mid-cast. At times I would end up accidentally cancelling a spell unintentionally but never had problems trying to cancel one on purpose. Moving to cancel a spell worked in FFXI, but I don't see it working here very well...
Why it worked in FFXI: Parties were settled into a single position normally thus you didn't need to run far. Just stand in range of everyone, tack on a buff, and resume play. One person ran off, pulled a target and the tank took over.
Why it won't work in FFXIV: For one, unless the durations are also changed, buff durations of spells. Typically a buff (Like Protect and Shell.) lasted for 5-4 minutes, up to 7 minutes if you used Roaming Soul. Unless this has been changed you'll find much more frequent downtime as you need to stop for everyone to stand still so you can buff them. Otherwise you'll just have to go a great deal of the fight without any buffs. The biggest issue is that camps tend not to be stationary, instead the entire party roams a large area continuously attacking creatures until they fall and then move onto the next one. Forcing spell cancellations by moving around will make it twice as difficult to buff somebody when the party is always on the move.
The only thing that could counter that is shorter recast times for buffs, heal spells, and longer buff durations. I worry if curing may be a nightmare when this drops. Need to heal with an AoE spell? No can do, gotta wait until the party stops moving long enough to do so without drawing a huge amount of hate from doing so as they engage. It's not really a fault of the dev team, rather the playerbase but doesn't mean it'll change anything but making a healer's job more difficult.