If what you are getting at is that Mercy is strong and unique, no argument here. But for general PvP, the metric that matters is (resources) per hour, which is a function of match speed. Every time you use a converter, you have to check the board, and while this becomes easier once you “crack” the board, first casts are costing you a bit of time, occasionally a lot of time on a bad board. This is why, to most people, Mercy is not going to be considered “dominant” - brainless exploder spam, despite catching a 5-10% loss rate instead of a 2.5%, is much faster and takes much less effort. I don’t need a well constructed team where I control the pace of the match to win, I just need to outpace or outperform the team I’m facing, which, given the value of a “turn” in todays game, is fairly trivial if you have the right troops.
Personally, I find myself more engaged when I’m playing battles with control troops where the decisions I’m making actually matter. That is most likely what you are feeling with Mercy being dominant - if you play like this all the time, there is no better opener. But I can’t deny that playing the same ones get old, and the game seems to be trending away from this. Most other fast openers carry a fairly large element of risk or are sloppy and largely take away your agency and will only lead you into battles where you aren’t going through the motions as edge cases because you didn’t get the right cascades a lots a troop but are still somehow in a position where you can win (and most people retreat even before then). I can’t help but feel that this is also intentional.
Actually, for pretty much every mode in the game, “perfect” play (as in, every decision matters and playing to the best of your ability the entire time) is not properly rewarded in any of them. In PvP, speed trumps everything. In explore, speed also wins and it is trivial, and raising the difficulty to the point you have to think about anything ever just gives you less rewards. Guild Wars punishes the use of “safe” play by penalizing taking a bunch of match 4/5s as an “action” each and lowering your score. Raids and Invasions give almost nothing for individual battles, and take exponentially longer if you are trying to play well the entire time (in my opinion, prohibitively long, which is a huge block in my enjoyment of them). Treasure hunt you need to be moving quickly or you’ll have much less rewards per hour, regardless of how “well” you play the mode in general. Even bounties, “good” play with a 3 bounty troop team requring a ton of careful play versus braindead exploder chains with two bounty troops sitting there like lumps only have a potential reward gap of one tier for any given buy-in while costing you many hours.
Vaguely influencing individual plays toward the direction of a “win” through instincts is just far, far more efficient long term, and likely remain as such forever. That is why Mercy is not a problem, and converter looping in general is not (and never was) a problem. It isn’t dominant strategy for things that most people care about (unless you literally can’t win any other way, but then there is still a drive to get the troops and setups where you can). If we see a swing away from the exploder meta such that converters are actually dominant again for consistent speed and safety and Mercy is at the forefront of that again, we might need to see some options comparable to Mercy (no, I don’t think she’ll ever get nerfed, nor does she even need to be). But I’m not sure if that would be good for the game based on where we are now.