Wouldn't hostile actions make you, well, Hostile? (Or at best Unfriendly if they were in a good mood?)
It's not like you can start shooting someone deliberately, killing them, and then their agency responds by declaring, "Oh, this guy used to be an ally, but he's now started killing one of our own now and is a potential threat to our stabiity and security, so let's just demote him to 'friendly' instead, that'll show him."
Destroying an Allied reputation through hostile actions is only going to make things worse for your rep in the long run.
I'd rather that rep gained for one major faction reduced the others by a similar amount. Perhaps allow multiple rep up to friendly, but above that the others start to fall back so that's it a juggling act to keep them all friendly and impossible to be allied to more than one.
I think that's exactly what the devs have now proposed for major factions with which you are not actively involved, except keeping the lower cap to Friendly (or more precisely, one or two steps above the cutoff to Neutral) for the major factions.
Minor factions to whom you are Allied, but not involved with actively, will remain Allied, as they know you personally and know what sort of CMDR you are, regardless of whichever political career you choose (as long as in your career you don't do anything actively and directly hostile to them).
As for major factions with whom you are involved, yes, I agree that maintaining Allied rep should continue, but what I hope the Devs can clarify is if there are sure fire ways of making sure that rep with a major faction you are involved with is maintained, and indeed showing
why there is rep increase, maintenance, or decay (in addition to what is happening at present, showing that change is happening somehow and in which direction through the blue and red arrows).
I'm hoping, or at least I think it would be a nice idea, that by pledging to a Power affiliated with a certain Major Faction, then your reputation with that Major Faction is immune to this passive decay as long as you remain pledged to that Power (sure, your rank could fall back with disuse, but not your reputation through disuse alone).
But wait. This then brings up the problem of inter-Power tensions, especially if both Powers belong to the same major faction: how will rep changes be meted out? For example, there will be problems if enemy agents of a rival power, who happens to be affiliated to the same major faction as your Power, interdict you within systems influenced by your rival, and you have to decide whether it is in your interest to fight them and hope your Power can justify your actions to the major faction in the background, or to flee, etc. Maybe if it involves tensions of an inter-Power, intra-megafactional nature, then overall rep for the major faction will be unchanged, but minor factional rep will suffer in that system belonging to the rival Power because of your direct action against the agents native to that minor faction.