Just as playing chess is possible with just a set of figures and a board, playing SteemMonsters is possible with just a set of level 1 monsters. You might also enjoy many fights against similar ranked players (more joy will come with more cards and more options) - But you won't be able to have success on a competitive level - Just like you won't have success at competitive chess tournaments by just enjoying games with friends.
I am not against calling STEEM-Monsters a P2W game - my point is that almost all competitive stuff requires a certain amout of investment in order to being able to compete on a bigger scale.