This could result in your opponents being attacked, or it might prevent players from attacking at alleither of which benefit the kind of deck that wants these effects in the first place
I do think that it is pretty likely that well see more of a red deck splashing into black or some other color than we will see a purely mono-red deck, but enough tools have emerged that I can still picture it as a single-color deck
in cEDH, Blood Pet is often found in Krrik, Son of Yawgmoth builds as a way to net an extra mana for 2 life, but also sees a lot of play in Inalla, Archmage Ritualist and Ad Nauseam lines to rush out these expensive spells or aid in comboing off with other cards to win the game
Its a bomb in Limited with the Survivor mechanic
When it ETBs, shoot things for 5 damage to kill multiple things
And how can you build around knights