Thoughts after some reflection...
The debate isn't if Known Spell casters should be permanently locked into their spell choices. They already had the ability to swap out their spells. The topic at hand is if they should be able to do so more often, moving from "once at level up" to "once after a long rest".
Is this too strong? I don't think I've seen anyone claim that, so no. Does it tread on the niche of the Memorized Spells classes? My opinion is that there's enough of a gap between "change one spell after a long rest" and "change all your spells after a long rest" to retain the distinction, though obviously not everyone agrees. Does it break their thematic class mechanics? As I said earlier, I don't see locked spell choices as a core element of the class story. It may define the playstyle of the classes, but I'm a lot more open to playstyle adjustments, especially when it involves making things less punishing and more fun. Am I missing any other arguments against?
Also, let's look at this change in the context of the entire rules packet. There's Cunning Action: Aim to make it even easier for Rogues to have Sneak Attack every round. Fighters get Maneuver Versatility to let them swap a maneuver after a long rest, similar to what Known Spell casters are offered, and Superior Technique to make maneuvers more accessible to non-Battle Masters. Monks get new Ki spenders. Spell lists are expanded. This entire rules revision is about opening up options and loosening restrictions. As part of that complete package, making spell swaps easier makes total sense.