You can have factions in a game without locking them out of vital abilities, such as answering all card types or having access to draw. In other games, it's normal, it doesn't led to all factions being the same or to RPS dynamics. In fact, it promotes diversity because your ability to play a reasonable game isn't limited because of the faction you took.I do not agree that all colours should be able to answer all types of cards. When all colours can do everything it often becomes unnecessary to use colours at all. For me there should be weaknesses and strengths with certain play styles. Strength in one area should be compensated with weaknesses. Otherwise you end up with rock, nothing instead of rock, paper, scissors (especially blurry, non-binairy, and more than three are nice) . Although it is funny to have a mirror match once in a while. It is not funny to only play those. Neither is it funny when you know you are dead before starting.
What they should do, is make each basic ability different between factions. Which they are already doing bit by bit. For example, creating treasures for ramp, fight for green removal and so on.