I think they are doing that wherever possible. Schools and ICUs moved underground where it is possible and necessary. In war time, you don't have the time or the money to finance big infrastructure projects unless there is a clear military advantage to them, e.g. reopening a bridge. And I wouldn't be surprised to learn that EU funds have already been used to strengthen civilian infrastructure underground in some areas of Ukraine.

Air intake would always be vulnerable, so no. Not really as a long term thing.

Bunkers are useful for surviving the initial attack, but not long term.

It might help. But, in addition to being generally unpleasant, it's also quite challenging to build things underground and also very dangerous to live underground. What if a bombardment or natural disaster caused the ceiling to collapse? Or blocked one of the many vents that would be required for air circulation?

It does help. It's one of the reasons the US is having such trouble with Iran. They spent 40 years putting important facilities 40+ feet underground. Which everyone knew before hand... Accept Rump 🙄

They would hit other critical structures not yet under ground. Power lines, water treatment plants, power plants, etc. You can't bury everything.

Building underground is very expensive and not even feasible everywhere. Going underground more probably wouldn't survive any assesment of cost versus risk. Not even Finland is yet 100% mole people.

Some finnish requirements were put in law after the wars. Ukraine could do something like that fresh out of the war, but good luck selling the idea to a country deep in peacetime.

Some things, like powerlines also need protection from the nature itself, but that's a separate concern and now I'm derailed, looking for a problem for this solution.

They they build Bigger bombs.

Bombardment! Bombardment!