You can't guarantee that.
Some people are ecstatic and/or amused about Piranha Plant, but others are
very angry and.or confused. And besides, comparing Piranha Plant and Isaac is like comparing Apples and Oranges, Piranha Plant is a recognizable Mario enemy (even if it's a confusing choice of character), while Isaac is a character who hasn't been in a game in 10 years.
You don't want people saying "okay, who the hell is this guy and why should I care?", introducing him as an Assist Trophy at least
introduces him.
Though, one could say that they should have instead just had him as a character from the start, but then we wouldn't have gotten
or
(there's no way they'd have not developed
,
and
, and
is a Semi-Clone).