It's very unlikely that the designated American Style Rumba tempo will slow down as time goes on.
Don't let the names confuse you. Unlike the Int'l Rumba, American Rumba is not considered a slow dance. It's a medium tempo dance with a somewhat upbeat and very rhythmical quality, probably closer to Cha Cha than anything else. It may have some lyrical Int'l Rumba-like moments mixed in, but the overall character is rhythmical.
American style's slow Latin dance is Bolero; This is the counterpart to Int'l Rumba. If you were to play an Int'l Rumba song for an American style dancer, he would dance Bolero. American Rumba would not work at that tempo, because it would feel lethargic and out-of-character.
American Rumba will probably never slow down, not only because doing so would water down the personality of the dance, but also because doing so would diminish the distinction between Rumba and Bolero. And what would be the point of having two Boleros in the same 5-dance event?
Regards,
Jonathan