The Manchester/Liverpool match is basically a unique match for all the wrong reasons. Racism, glorifying songs about the dead, violence and thuggery. There is no fixture that can quite live up to it. There's an Oasis song that says something like "because we need each other, we believe in one another" which I think underpins it. Increasingly now they really do need one another as their image is increasingly built around violent opposition. As an Everton fan I'd love to see Liverpool relegated but I think United fans would miss them as it's a frame of reference.
As I indicated in the post above there is a material base and similarities to both clubs behaviour. Both clubs have fostered the idea that they are special as above the rules. If you want to stop songs about Hillsborough of Munich you need both clubs not just to condemn those songs (which they don't do enough) but also challenge the entitlement of both sets of fans that they are better than the rest.
As they both slip into relative obscurity you can see the anger of both sets of fans and a pride at being hateful coming through. For LFC fans being the most hateful towards MUFC (or vice versa) becomes a trophy in itself. Perversely though you need the other club to be seen as the biggest and best in that story, as nobody would hold anyone in great respect for building a narrative around hating Fleetwood. They both have identical views that they are conquering the great superpower and caste themselves in the David role against Goliath.
The only final point is you will see them starting to fetishise the things that make the match unique. Once this was because one of them were the best team in the country. Now that's patently not the case they will need to find something else. I suspect it will be all of the things I first mentioned, the racism, the violence, the scant disregard for human life. No other fans will stoop to their level so they will revel in it. You can already see it and then the slightly more intellectual crowd will condemn the opposing team while at the same time feed into the exact same conditions that lead to the behaviour of their own fans by telling them how special they are, what an amazing club they are and what an amazing match it is.
It is the sort of logic that leads to working class youth traipsing round with groups like the NF/BNP, a glorification of the worst bits of humanity. Lets be honest we know that in both cases that's essentially the social base of much of their support base. Narrow minded little thugs who want a punch up.