Comparing Rotom-W to Pex is pointless because they, even with their shared Water typing, perform very different roles. Sure, they both mostly sit there and burn stuff while not draining too much momentum due to respectively Volt Switch and Regenerator but the mons they check are very different. Most notably, Rotom-W outright counters a lot of Ground-types while Toxapex gets wrecked by LandoT, Garchomp, Excadrill, Zygarde (needs boosts but still) and the overhyped Mamoswine. Their archetypes differ too; Rotom-W usually fits on VoltTurn and more offensive balance teams, while Pex is mostly a Stallmon that can also contribute to the defensive backbone of Balance and Hazard Stack teams. and offensive teams that forgot to update after the Pheromosa ban
Washtom will likely drop soon (aka RIP my TR UU teams) but you can't say it's outclassed by Pex for much of the same reason Ash-Greninja isn't outclassed by Pex; they perform different roles.