I think that not having an anime and remain "obscure" is for the best, it won't make the manga get "cancelled" in my opinion, after all, manga that are cancelled are usually pubblished in MAGAZINE before being printed out, and people still BUY that magazine... so POTENTIALY, if the mangas are good, they won't get cancelled.
With that said, of course an anime can affect the sales of that manga, and probably because it make it better known to the people, INCREASE the sales of that same manga.
But STILL, it will have the same effect a live action or movie or games could have on it... its just, ONE OF THE WAYS that one have to make it better known... and for that reason, i think it should be done WELL if it has to be done, because if not, it have the same effect of a tv commercial...
So in my opinion, a bad anime is just as a bad commercial, it makes the cut to make the product known, but that doesn't assure good sales, a good anime could surely make better sales, as a good commercial does, but it will probably make more sales on the anime side still... so i think it's better if the anime is done well, but not for the sales of it... just because it can be appreciated by fans and non fans of the manga... while a bad anime adaptation? well... it will make a fan drop out from something he likes, like the recent ubelblatt anime adaptation... i had already read all the manga and i really liked it... but the anime? well... i dropped it after a few episodes... it doesn't do the manga justice at all...