As someone who drives in Wales a fair bit, it was badly implemented so it became unpopular.
The blanket rule was applied to roads it shouldn't have been, with a slow and costly process to fix the speed limits on roads that's shouldn't have been covered.
It makes sense on all side streets, but it doesn't make sense on all main roads. Some yes, but not all inside towns and cities. I think this is being unpicked it seems with more flexibility for councils? I don't really follow Welsh politics, like many people I suspect.
The other issue is enforcement or lack there of. There seems to be zero enforcement so you get into the situation of driving down roads and everyone is still going at 30 ignoring the law. It puts you under pressure to go at 30 and it's easy to drift up to that speed.
I'm am generally a supporter of the new law but the politicians have to take ownership that the reason it's controversial is because it was poorly implemented. Its easy to paint the critics as extreme or as part of a "culture-war" but that's just people taking advantage of actual anger and frustration.
The policy can be popular I think - there just needs to be some minor changes. As an example I can think of 4 roads in the town I drive or walk on that could do with going back to 30mph; thats nothing in the 100s of roads in the area.
It'd even potentially be safer as people are just breaking the law and speeding on these road anyway making it less predictable for pedestrians.
An example is a long main road that climbs up a steep hill in my town. It's actually a struggle. climbing it at 20mph, and I even get foot pain trying to keep the accelerator at just the right depression to stay at 20mph. The road is wide too so you're struggling all the time with the accelerator, monitoring your speed as it's natural to go faster on wide roads and other drivers putting you under pressure to go faster. People are even overtaking each other which can be dangerous as you don't always see what's coming down the other way easily.
So I'd be worrying less about the fringe lunatics stirring up anger and more about tweaking the implementation to get the majority on board. That'll take the support and interest away from the fringe noise makers.