I guess I just have a positive outlook on the series, or I'm blinded by hope aha!
Ether way, to me the fact that they are using the characters still is a positive, I'd personally be a lot more worried if the series was completely dead, in a sense that even their characters were not appearing in anything at all!
Also, PS have given a lot of hints towards a future installment. Saying things like (not directly quoted, from mind) "the soul still burns" "exciting news in the future" (or something similar to that, I can't remember) "we can't talk about the future of the series... Yet" and other small little hints. I admit though, those hints were around from Lost Swords release to about last year, I think. Since then, they have fell silent, which is worrying.
Anyway, to me I think the fact that the characters are still living on in other games is an indication that they still have interest in the series, and are most likely just trying to figure out what the best step would be to take the next installment.
Only time will tell, however a lot of people are losing interest now because it's been so long. If they do have something planned, a company can only hold interest for a certain amount of time, and putting your characters into different series to garner interest isn't always the best move, and people will get bored. Guess we just have to wait, although even I am getting annoyed with waiting now. I'd rather know if they are or are not planning something for the future, and definite answer would be better rather than little hints. At least that way, if they say that there won't be no more installments, people can stop waiting and wishing for one.