There's certainly a lack of advertising for Grepolis, compared to what there used to be. TV advertisements, website banners, etc. Recently, the only advertising that I have seen for Grepolis is a little still image hidden in inconspicuous places on a couple of mini game websites. In contrast, I see frequent advertisements for Warlords of Aeternum (Inno's newest game, and app-only, so far as I recall) in-game when playing other app games.
I think finally bringing out Domination was an attempt to increase player retention (for those bored of World Wonders). However adding a new, different endgame could also have been a great lynchpin for advertising Grepolis to bring a new influx of players. Unfortunately, they either missed (or decided not to take) this opportunity.
I really hope that Inno is not in preparation to close down this game (and by extension, it's community), but the evidence of decline is there.