Tell that to milions of chess players around the world who are playing with ( and against ) the same 16 fugures all over and all over again for tousands of years. Or to millions of poker players worldwide that are playing with the same set of 52 cards all the time. Have You seen how popular chess and poker are in compare to gwent?
My point is, that in games where You play single player campagins (like witcher 3 or many others) the diversity is very important: new items, new areas to Explore, new monster to slay, new puzzles to solve, and that is what kept You playing and is a root of Joy from the game: diversity, and more and more of it.
But in the games where You face in one on one "life or death" combat another real intelligent opponent, not AI, the real source of Joy is when You are able to beat him with Your flair, sharpness of mind, Intelligence, creativity, and out-of-the-box thinking. And in the reverse, if You loose with another player, its often source of frustration, because subconsiously You feel that that guy or gal, or maybe just a 10-yo kid with iPhone it its hand, was smarter and more cleaver than You and he ruthlessly exploits all Your weaknesses and mercilessly beat You to the death in the game. And that is why chess or cards are so popular games - because not the diversity of cards is important, but your ability or inability to use it better and smarter than person on the other side of the table or screen. And that is a FACT, what mentioned above millions of active players of - amoung many others- chess or poker ( games that didn't change a bit for hundrads of years ) all over the world prove. Of course You can have your OPINION that diversity if most important thing in card games and let's leave it as it is than because it waste of time to discuss it further. But Your OPINION don't change mentioned above FACT at all
In conclusuon: reject opponent button or filter of opponents is necessary. But no to eliminate often encountered factions to make game " more diversified and less boring" , but to make up for poor gwent devs team ability to balance the game and geave players option to just eliminate from metchups unfairly strongly designed factions or leader abilities, regardless of their popularity in current meta.