I saw already some teams full of legends.
I think it's risky, with this IV reroll, the incoming recatch, etc.
I'm here since may 2016 and I remember PRO staff saying they don't want team of full legends, because that's not how PRO works.
I think that, after the latest changes and the incoming one, a new PVP rules should be in order: a player can only use max 2 Legendary in a single PVP match.