Tampa didn't have a ton of young talent at the time. Conacher was having a very productive season and the geniuses of hfboards skewered Yzerman when the trade went down. There was about a year there after the bolts made the conference finals when the hf group think started the Yzerman is over rated to terrible as a gm meme.
There wasn't? Johnson was in the late stages of his AHL MVP season, Kucherov was already in NA and dominating the Q, Panik was highly regarded back then.. Alex Killorn (still one of the best kept secrets back in his prospect days IMO) was already having a monster pro rookie season. Palat was already NHL ready, though everybody thought he would become a thrid liner.
Obviously it wasn't what it turned out to be back then, but still there was plenty to choose from in terms of forward.
Yzerman was wise to trade the hot guy everybody was talking about, who hadn't produced for a stretch of games already. He later said that he was willing to trade Conacher because of Johnson. His scouts and himself saw more upside in Johnson.