A lot of them are not maintainable - eg, there's no economical way to "fix" reinforced concrete with an incorrect blend or incorrect reinforcement.

Is the cost of demolishing and rebuilding it so much higher than building it from scratch?

Yes, for example, it is much harder to rebuild a bridge in the middle of a city than it was to build anew. Maybe that bridge was constructed in the open field and now the area around is built up. There are a lot of traffic and it should go somewhere while the bridge is closed. A lot of communication lines could go through this bridge and you have to account for it, plan for it and somehow move it while you’re rebuilding the bridge.

Can you just leave it as it is? I’m not sure, it’s a Soviet-build bridge, lots of cement was stolen during construction and now some pieces of concrete are raining down.

This exact problem happened in Montréal with the Champlain bridge, cheaply built in the 60s. A new bridge had to be built at the same place (inaugurated last year) and connected to existing infrastructure.

But wait, doesn't that depend on whether you view the money not made during the shutdown of the bridge as "lost" or as "not made"?

The time spent rebuilding and not getting value out of the thing should be included

It seems counterintuitive to conclude "we can't afford to build this bridge in the first place, because it would be too expensive to temporarily close it". But based on your sibling's post, maybe you're right?

Yes, especially if it was made with materials which need to be abated

Often economies run on star cities, and if your country's economy is disconnected from those star cities because of a breakup, your country's economy will also suffer.

There was also the entire communist central planning thing that made resources misallocated.

Thank you for an explanation that makes sense!

What happened to the Soviets in the end?

What kind of logic is that? I'm not trying to defend the USSR here, just understand how infrastructure spending works. But if that's how you conduct your reasoning, wouldn't you also believe that Abraham Lincoln must have been wrong about everything, seeing as how things ended for him?

The Roman Empire isn't around either, but I don't blame their infrastructure projects for that. More likely that those helped them last as long as they did.

The USSR collapsed because it overspend on pointless projects. Bridges to nowhere or military bases in Afghanistan add nothing for the economy. Just because it's infrastructure doesn't mean it's good.

So this seems like a roundabout way of answering the GGP's (my) question by saying that the post-Soviet countries are only declining to pay for maintaining those bridges that happen to go nowhere, which is presumably fine, because nobody needs them. The Soviet-era bridges that do see frequent use, they can afford to maintain/rebuild just fine? Are you answering this from an epistemic position of particular knowledge and experience about Eastern European economics and infrastructure, or are you just speculating that this is the way it probably is, based on your opinions of how the world works?