Successful payload defense can only be considered a stalemate if you consider the term to mean when neither team can successfully “push”, or when the objective doesn’t move. However, the term stalemate more accurately describes a situation in which neither team can accomplish their objective, but in the case of payload the objective of the defenders is to prevent the cart from being capped. Thus a true stalemate is impossible in stopwatch because one team is always succeeding, the cart is being capped or is not.
The advantage of payload in making this static sort of gameplay still somewhat interesting is in giving the attackers a massive advantage in spawn wave timing, allowing them room to take risks. The same feature does not exist in 5cp, the attacking team is always putting themselves at a disadvantage, because unlike in payload they do have something to lose by taking risks: their last point.
This change is effectively the same as decreasing the 10 minute in game cap timer, as both simply create more reset opportunities in case of stalemates. A lowered in game cap timer has also been discussed in 6s, mainly in the context of the euro config/playstyle, but I don’t remember where that ended up going.
At the end of the day I don’t think reducing the time maps are played would make the gameplay any better beyond potentially creating more “oh shit we are down points we need to make something happen” moments, but the possibility of 3 potentially never ending golden caps as opposed to one could actually be far worse for the gametype.