I'd say the length of time between the sequels is more evidence that they're playing on nostalgia. And the MCU does have endings. Cap and Ironman are done and have passed the torch. T'challa doesn't seem to be getting a recast either. Logan appearing in this doesn't mean he's going to be in future films.
Time actually works against recasts due to nostalgia.
And I think you're expectations need to be tempered since we not in the beginning anymore. However there are still new characters being introduced.