I'm not here to start a debate or attempt to change in minds, but I will give my perspective on this.
I don't understand why there is this perception that just because Wakanda has never been defeated that it means they CAN'T EVER be defeated. Yes, they were undefeated, until they weren't. I think that DoomWar made it very clear that the Wakandans had grown prideful and arrogant, and like with all great nations they get that mindset, defeat inevitably happens (see Empire, Roman for details). Every great champion has known defeat at one time or another. Its not the fall, its how you overcome your failure, learn from it, and bounce back from it.
Thing is, every single fictional nation in comics always eventually faces such adversity: Asgard (also previously undefeated), Attilan, Atlantis, they've all gone through a period of upheaval, and they always come back -- and Wakanda will too, all the stronger for its trials.
Secondly, although T'Challa certainly is revered by the majority of Wakandans, there has always been a segment of the populace opposed to him. Its been portrayed in almost every Panther run starting with Don McGregor's, Priest's and Hudlin's. I believe he was even asked to step down in the 4 issue limited series by Peter B. Gillis and Denys Cowan.
T'Challa's frequent traveling to the U.S. and abroad is perceived by some of his subjects as evidence of disinterest in the everyday business of ruling Wakanda. So while he enjoys great popularity at home, it has never been universal.
Replacing T'Challa as Black Panther is hardly anything new. Priest replaced him with Killmonger (albeit briefly) and Kaspar Cole. I can live with T'Challa not being the Panther for awhile -- partially because I know he'll eventually reclaim the title. What I DON'T like is when he's pushed off the stage and unseen. That hasn't been the case here. Even though he's not the Panther, T'Challa has still been very much in the spotlight. Compare that to the Kaspar Cole run, where T'Challa often didn't even appear at all and other times only briefly.
And wow, replacing characters temporarily with someone else is certainly no phase. Its happened to virtually every high profile hero in comics, both at Marvel and DC. Bruce Wayne was replaced with John-Paul Valley, and more recently has been replaced with Dick Grayson as Batman. Steve Rogers was replaced with John Walker, and even now is currently being replaced by Bucky Barnes. Superman was recently replaced by Mon-el. Barry Allen was replaced by Wally West who himself has now been replaced by Barry Allen. Hal Jordan was replaced by Kyle Raynor who has now been replaced by Hal Jordan. Peter Parker, Matt Murdoch, Tony Stark, Princess Diana, Thor, -- the list goes on and on, and probably always will.
I myself have misgivings about T'Challa's new role, but I certainly understand why they've done it. Tom Brevoort said there's a perception from some fans that the Panther is on the periphery of the Marvel Universe instead dead in the center of it where he belongs. I don't know if this new approach will work, but if it gives T'Challa more exposure and raises his profile among fans and in the Marvel Universe, I'm all for it. I remember a time when T'Challa spent most of his time in comics limbo, and I'd certainly rather see him in Hell's Kitchen than in there. My two cents.