The linear approach in the "second half" of the game isn't the problem, they just didn't implement it in a way that would truly benefit the player (story- and gameplay-wise). Having that said, I'd prefer your solution any day, keeping the open world aspect of the game intact just makes a lot more sense and has a lot more potential.
The tedious chapter, however, seemed like a lazy and cost-effective way of increasing playtime without disrupting the story to a degree it wouldn't make sense at all.
They should have introduced the ring as an addition to your arsenal and a necessity to combat the daemons' ever-growing strength. Instead they cut your powers, separate you from your allies, introduce (non-combat) QTEs, stealth sessions, enemies that can disrupt your ring "powerful" abilities with ease etc etc, all resulting to a longer chapter.
Ardyn intentionally blocking your powers had no meaning (hell, a man-made machine blocking divine powers doesn't even "make sense" IMO). Ardyn kidnapping Prompto had no meaning. Being separated from Gladiolo and Ignis did make sense (in a way) but them finding you at the perfect spot at the perfect time with absolutely no way of tracking you down (the citadel is frickin' huge)? Yeah, right. But at least Ignis didn't hold you back that much like previous chapters where he was tagging along, right?
To me it looks more like a "convenience" than a poor interplay between gameplay and narrative, at least for this chapter. For the rest: the narrative is all over the place, so I'm guessing they changed it a lot (makes sense considering the game's development history) and gameplay had to to adjust accordingly.
