The recast of Meisui, not TCJ. The TCJ recast can stay 120s, obviously. But the combined replacement period plus recast for Meisui has to be no more than 120s because Meisui needs to be usable again at the next TCJ.
What QoL does it serve to keep them two separate buttons though? Suiton only lasts 20s, so a 30s replacement period covers the entirety of Suiton's duration. A longer replacement period or a separate button just allows someone who did the wrong triple jutsu sequence to reclaim a tiny bit of lost DPS by spending a real mudra charge on an extra unscripted Suiton. In the overworld things die so quickly it doesn't make a meaningful difference if you're forced to live with a Raiton instead of a Suiton+Meisui+Bhavacakra, and on dungeon trash you're not going to use single target jutsus until there's just one or two targets with low health anyway.
I just don't see the opportunities to use Meisui separately to be worth more than the convenience of marrying one two minute action to the other two minute action it's intended to follow.