The indication has always been that ending your turn on the top of the stairs should generally not result in an Abyss situation. In practice, there may be exception to this given extremely chaotic behaviour (I'm thinking PC has <10 health, is massively poisoned without resistance, prayers or potions), but this would have to be determined at the discretion of your league mod. To date, no such situation has arisen that has not been successfully cleared. The only situations where the rule has been applied to date were situations where no player was successfully able to descend because the difficulty of the level spawned made further progress impossible (in the two precedent cases, both had extremely difficult tension rooms blocking the stairs).
In this case, nothing in particular has made this situation so difficult it could not be passed. You've all just fallen to the wrath of the RNG.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.