I definitely agree with this. Furthermore there are options to explain what happened. I recently rewatched Black Panther and T'Challa was very nearly killed. He received the Heart-Shaped Herb and became the Black Panther again but it wouldn't be too hard to use that as an explanation - T'Challa wasn't completely healed. Even Wakandan medicine wasn't able to repair all the damage. T'Challa was on borrowed time since then. Thus why he was so focused on changing the culture of Wakanda that resulted in all the problems his people faced in that film. If he was expecting another 3 or 4 decades of active life (which he should have gotten barring injury or accident) he would have had time to make reforms. If he wasn't expecting to live very long he would need to make more drastic changes. Oh, and for bonus points he could take the blame for any problems knowing he'd literally take the blame to his grave and give his successor a better position to deal with said problems.
Or not. I'm hopeful that Disney can come up with a good story-related reason for T'Challa's passing. As for characters who can step into the role of the Black Panther there are:
- Nakia - T'Challa's former love interest (and apparently again at the end of his film) but who turned it down for being a spy and not a warrior. Having her training into the role with Okoye could be interesting to watch if done well.
- Okoye - As the head of the Dora Milaje she's been shown to be a warrior and soldier in her own right with a strong sense of loyalty. Okoye is a logical candidate especially if Shuri ends up on the throne.
- Shuri - The young princess is more of a "Tony Stark / Iron Man" or "Spider-man" style hero (technology, intellect, and cunning) but has been shown able to fight.. with more technology. She was the champion of her tribe during T'Challa's initial challenge series. She might have refused to challenge him but I doubt she'd have gotten that role without being able to at least put up a fight against someone on T'Challa's level.
- M'Baku - While he is a very traditionalist leader, M'Baku has Wakanda's best interests at heart. (Behind his own tribe's best interests, but still..) M'Baku stepped in when Killmonger decided to ignore tradition and simply have T'Challa killed. He is a skilled warrior and there's plenty of cultural threads to explore as M'Baku clearly does not worship Bast nor the legend of the Black Panther.
So that's four characters off the top of my head who could step into the role of the next Black Panther. And all four characters give the MCU team a great way to show their respect for Chadwick Boseman by mourning during the film. (Or at least remembering him.)