I have a different theory, one that the Witnesses will hate. Perhaps Matt. 24:14 and Mark 13:10 are not fulfilled as yet. These point to a future fulfillment:
Matthew (24:3) speaks of “end” as συντέλεια (τοῦ) αἰῶνος the end of the (present; αἰών) age = a combined end, a point of time marking completion of a duration, orig. ‘community contribution’ [the basic semantic component relates to something complementing something already present] in our literature only in a temporal sense and in v. 14 τέλος = the last part of a process: “This good news of the kingdom will be proclaimed in all the world as a testimony to all nations. And then the end will come.” The proclamation of the kingdom is therefore to start on the eve of the Tribulation. The preaching campaign is one of a kind, not to be repeated (cf. Matt. 24:14, 21 CSB). See BDAG.
Mark (13:10) is even more specific: “Also, in all the nations the good news has to be preached first.” πρῶτος = pertaining to being first in a sequence as to time, set (number) or space (BDAG), in the sense of a precondition in the Synoptic eschatological discourse (EDNT). The preaching of the gospel is a precondition and should therefore commence before the Tribulation takes hold. It is not to be repeated (cf. Mark 13:10, 19). See also unique use of πρῶτον in connection with “judgment” in 1 Pet. 4:17.
In my view Acts 2:16-21 was only partially fulfilled during the first century. This also points to a future fulfillment.