Without wanting to justify too long absences, a slow down is not too unusual IMO. Unless you have everything written down beforehand - in which case there probably also was a slowdown, just invisible to the players - you will start off with all kinds of ideas and very enthused and later on have to go into the gritty details and enthusiasm breaks down. Everybody has probably experienced it in some project, even if it only is redecorating the bedroom or writing a small program to sort your Pokemon cards or what have you.
Surely this is an individual thing, and quite a few devs manage to keep a steady schedule without sacrificing update size or quality, while a few even may speed up once they reach the final stretch and everything is laid out, but I am not at all astonished that quite a few games slow down over time without it having to be some intentional stretching out.
