Even then, Romero himself often goes out of his way to show that the Zombies aren't much of a real threat at all. Two guys armed with nothing are capable of effortlessly running through hordes of the things and are able to simply shove them away if they get too close. The bikers near the end even start toying with them as they rampage through the mall. People only get infected when they get reckless (Roger), or as a direct consequence of trying to fight one another (the bikers, Stephen, etc). Basically, Romero's zombies are only a threat because the people refuse to actually band together and stop them.
I think the point is the infection caused the outbreak, but then stopped spreading. So everything goes to shit really really fast, and is basically unrecoverable at that point. However, most of the threat is over by then.
Hell, even Night of the Living Dead, if taken into it's own continuity, shows that a band of twenty-something hillbillies with no military training and armed with rifles are perfectly capable of taking care of the problem when it initially breaks out.
But yeah, the later Romero movies (Day, Land, etc) basically show that they're basically unable to recover at that point because there are simply too many infected for a band of hillbillies to stop.
Plus it's not a clear cut 'infection' in the Romero films - there's nowhere to quarantine because EVERYONE rises, whether they're bitten or not, and infection is just one of a bunch of explanations (some scientific, some supernatural) that people come up with. The whole 'you get bitten, you turn into a zombie' plague concept came from Fulci's unofficial Italian-language 'sequel' to NotD 'Zombi' (where it clearly starts on an island and spreads, whereas in Romero it's everywhere and unaffected by cause of death). In the Romero films getting bitten is a death sentence, not because that spreads the infection (if it's an infection, everyone is ALREADY infected), but because you're being bitten by a rotting corpse (c/f a Komodo dragon's bite).
So it's a permanent shift, no matter what they do. The early parts of Dawn suggest that they've cleared the city of undead several times over already - but the kicker is the city isn't adapting to the permanent change (they can sweep and clear the place, but nothing is being done about the hellhole slums full of people too impoverished and suspicious of the government to cooperate, half of whom can't speak english or read the warning pamphlets while the other half are actively hiding the elderly and ill while fighting the troops to stop them taking away their grandparents).
The whole concept of a zombie pandemic caused by infection requires a lot of logical constraints (going beyond thoes to do with the zombies themselves, obviously). A straight up infection (starting with a patient zero, then spreading outwards) would follow the same vectors as any outbreak, and with zombies being a lot less effective at spreading disease than an airborne illness or an STD (people can see and tend to avoid zombies, plus they can't near-instantaneously infect everyone in a large room), there's no way it could reach pandemic status.
Funny thing is, 'fast zombies' would be even less likely to cause a pandemic - they'd fuck up one extended urban area and then stop as soon as they hit the first significant natural barrier or gap in human population. It would have roughly the spread of an ebola outbreak, where entire towns get wiped out, but minimal global penetration. There's a reason why the Arican super-viruses like Ebola and Marbuhrg aren't pandemics despite being massively infectious and with close to 100% mortality, while AIDS is. It's because that 90%+ mortality rate occurs so damn quickly. A disease needs a host population in order to spread - HIV does that really well, because it kills so slowly that someone can infect a lot of people over a long time period (and hence greater geographical spread) before showing symptoms. With ebola, you're bleeding black blood out your eyes and nose within hours - it will wipe your village off the map, but because it kills everyone so fast it burns out its host population before it can spread to the next town. Fast zombies would have the same problem - everyone would die very very quickly, until the zombies hit a gap in population and have nowhere to go.
Now if you really wanted an unstoppable zombie pandemic, you'd want rats, birds or mosquitos to act as a host
. 28 Days Later almost does it - rats and pigeons are both infected, but they're also turned aggressive just like the primates. That's going to limit its spread - they're both highly social creatures and so the virus will just eradicate the local rats and pigeons like it does the humans. In its favour, 28 Days Later gets that right - the outbreak is confined to the UK (possibly even just to England/Wales), and doesn't reach the farm they're on at the end (which seems to still be in England). Of course, that makes the soldiers a bunch of over-reacting whinypants who typify the English culture of considering anything more than 10km from their hometown to be 'long distance travel' ('Well we could just go to Innverness - I mean, there's still rural parts of England that don't have any infected, so there's got to be tons of Scottish towns that are completely safe. Wait, that's a 14 hour drive??? Fuck that shit - if the infected are too lazy to walk there, I'm too lazy to drive it').
