That leaves out the other side: who agreed to the deal and dishonestly pretended magic free money would pay for it? The unions didn’t force the state to make tax cuts below fiscally prudent levels.
> When one party is funded by the public service unions
At least in the U.S. we don't have parties which are funded by unions. The unions contribute, and get out the vote, but that's rarely uniformly going to one party which gets a veto-proof majority everywhere. You see divides — e.g. the teachers unions lean Democratic but the police, firefighters, and prison guards lean Republican — and a lot of local politics showing counterexamples for any of those trends.
It's also not really the point I was interested in, namely that several generations of officials choose to cook the books so they could make politically popular moves without raising taxes or even cutting them. This is not a problem specific to pensions and it's definitely not limited to a single party as e.g. three decades of Republican magical thinking about tax cuts paying for themselves should demonstrate.