Bowser with Dash Slash is a whole different character, but not necessarily a better one. Klaw was always one of our most valuable moves, and I think we went overboard on Dash Slash hype pre-EVO 2015 because we were all still so sour over the new Bowsercide results. Patch 1.04 was by far the worst few months for Bowser mains. Nowadays, Klaw has a better hitbox, can be controlled fairly, and serves a more valuable niche of a grab that lets us kill at % ranges where Uthrow to Uair isn't guaranteed.
If we entered a customs on meta, Dash Slash would still show up for our recommended sets, but perhaps left for specific matchups, or just people that want to play a less-effective heavyweight version of Sonic over default Bowser. For instance, Dash Slash would be a nightmare to get stuck with against Bayonetta. It has as much windup as the average smash attack, so anybody with a counter has that sort of easy answer. A waveslash can be as safe on shield as Sheik's Fair in terms of block advantage, but we're not shiek. We don't have a 2 frame jab or 6 frame grab to keep up shield pressure. And while it's hitbox is massive, I wager any sort of sword character beats it with attacks, rather than trying to block. I love its recovery and landing mixup applications because Bowser gets predictable when we always retreat to a ledge. Plus a short hop Dash Slash gets us really damned far offstage for an aggressive edgeguard. We can meet Sheik with a Fair before she reaches vanish distance of the ledge, that's not reasonable for us to do in Customs Off because we can't move that fast horizontally offstage.