When Gwenom attacks, she unlocks the ability to play cards from the top of your library for the rest of the turn, paying life for their cost instead of mana
First, if your sideboard is full of silver bullet creatures / hate creatures against specific match ups, you can get rid of a useless mana dork in favor of your tech in game 1
And thats before demand has had a chance to drive prices up even further
How much worse is this for mono color decks than Blood Moon or even Spreading Plague
They will sometimes fix issues that get enough mainstream attention (Ixalan's Binding exiling Squee for example), and the more judges and players show they're unsatisfied with the current rules, the more they'll try to live up to their claim of covering every possible corner case
In the above comparison, the commercial release of the Latios card is on the left while the promotional one is on the right