Hmmm... I see that "idea" is among the translations of "pojęcie" on Wiktionary, but generally I'd say its main meaning is "notion". The only situation when I'd surely translate "pojęcie" to "idea" is "I have no idea" = "Nie mam pojęcia", but that's more of an idiomatic thing.
We want the subject of the sentence (here: of the clause) to remain the same, you changed the subject from "this idea" to "this/that/it" (... that [this/that/it] is a worse idea).
I probably wouldn't think of it, seeing the English sentence, although sure, we translate "chyba" to "I think that". But that "I think that" is close in meaning to "It seems that"...
Let's see... "Chyba ten pomysł nie jest gorszy", that would be that. I can add it, I guess.