Why are you making this distinction between 'traditional film fans' and 'MCU film fans', the MCU movies are traditional films lol. What you're describing is how simply how sequels and spin-offs have worked for decades.If you remember the ending of the first Black Panther, in the after-credits scene T'Challa made a declaration in front of the UN in which he revealed the true status of Wakanda to the world and calling for "looking after one another as if were one single tribe".
In the last scene of the previous film we saw T'Challa using Wakanda's resources to help youth as you correctly pointed out. Between the last Black Panther film and this one Infinity War happened, a battle in which Wakanda was the last line of defence for the entire world. In the new film we see an example of wakandan outreach centers. You are right that we don't know more details about the full extent to which Wakanda kept its promise, I suspect will get more details in the Wakanda series that is planned.
I know that traditional film fans really hate this sort of answer and I get it, I really do, but they have to come to terms with it. Themes and plot lines being continued in other films and series is a feature, not a bug.
Anyway, I think you are misrepresenting the battle of Infinity War in favour of the argument that Wakanda somehow is a world protector. The battle in Infinity War isn't set there because Wakanda is the 'last line of defense for the entire world', it's because in-story it's the only place where Vision can go because only Wakanda has the tech to remove the Mind Stone (which ironically falls in line with my original remark that Wakanda clearly didn't share their knowledge). They don't lead the enemies to Wakanda because of morality or the idea that they are the world's last line of defense, the battle happens there because that's where our heroes happen to be at that moment.
Also, the framing of the final battle as 'the last line of defense for the fate of the universe' feels a bit strange even if it's technically true considering how much the movie kind of undersells the gravitas of the battle. Like, outside of the people being at the battle nobody is seemingly even aware that there's an invasion happening. The few people we have seen getting snapped were just going about their regular day.