Sonic and Silet Hill were both ruined by the creators of the games, who are the worst of the fanbase. Americans began making Silent Hill games after the Japanese stopped due to being fans of them, and as a result made absolutely shit games. Sonic suffered the identical fate.
People claim Sonic fans are bad, and constantly go overboard about the new game before ignoring it once they realise it isn't any better than the previous and begin going over the top about a new game again. That may be true but you have to remember that the fans have been begging for either no more games or a good game for the last 18 years (with adding Sonic Adventure 1 and 2 to it as the best recent games) Yet all they get is another shit installment by the "fans" who make the games, trying to appeal to a fanbase, and fail yet again to understand what people actually want / don't want.
The original Silent Hill creators stopped making games, which was good as they wanted to stop it. But the problems came when foolish American fans decided to take it upon themselves to bring the series out of its grave and make the games "for fans" thus ruining the series from that point on. It was identical to the original sonic creators, and for a lot of rebirthed series for that matter.
There is nothing worse for a series than to attempt to force life back into it when the original creators have left it long ago, it will never be as good, it never has done and never will be. By all means continue to enjoy them and make little fanmade projects etc. to enjoy it. But don't attempt to revive a game series when it has already run its course.