Brothers, its simpler then you think. I just ask you to ask yourself when Allah says... “Remember when Allah took a solemn promise from the prophets...” Were all these people who are being addressed, there when Allah took the oath for which Allah SWT is asking them to remember?
Of course not! And I hope you don't think they were.
Whatever oath Allah SWT took from the prophets was in form of revelations which were given to the people by the prophets themselves, taking form of scriptures for each nation/ahle-kitab(the main addressees here) to bear witness to...and which they deliberately not preserved but knew about very well!
So ask yourself again now where did Allah SWT told the last Prophet SW in his revelation that there is going to be another messenger after you!
Hope this will help you understand.