The important thing to consider is that Spencer is current writing ASM #832 (TASM #31 V.5). By the end of 2019 that will be #837. At the rate of 24 issues per year, if he stays for three more years that will be #899, and of course Spencer will stay for the milestone issue of #900 at least having come that far. But after that, it depends.
That means the writer who comes after Spencer has a shot at doing ASM #1000 which will be the first Marvel title to make it that far as opposed to the pretend Marvel Comics #1000.
So it's got mad prestige tied to this.