I don't know why we have that rule, but it seems to go way back. Only new elections I can find are when voting rights has gone through a large extentions in the beginning of the 20th century. As constitutional reforms needs to be voted on in parliament before and after a eleciton, parliament has after voting through the new voting rights dissolved itself and a new one has been elected with the new electorate to serve the rest of the term.

Ancient traditions, I guess?

I don't think the centre party will pay a political price. They are indespensible to forming a government, so they won't pay a price in parliament. And I doubt they will in the election either. They are an amalgam of the farmers party in the country side and young neoliberals in big cities. In the country side deregulation of rents isn't a big issue, because with ongoing urbanisation to the major cities, small towns tend to have low rents and a problem with getting tenants. So their voters there aren't supportive of the issue, but I doubt it is a dela breaker either. And the young neoliberals are obviously convinced that deregulation is the solution to all woes.

With the Liberal party polling to low to attract support votes and the Christian Democrats and Greens balancing on the 4% limit to stay in parliament, the Center party might very well mop up one or more parties in the centre, especially if there are two elections in a row. The Center party, the Moderates (that are conservative) and the Social Democrats are the three well-funded parties that should theoretically gain from exhausting opponents election funds. But that is theoretical, cause we never have new elections, so we don't know how the electorate would react.

Or actually we have new elections. But only when vote counters mess up. The votes are counted twice (election night and then automatic re-count) and if differences emerge between the counts that would move seats, the election is re-done. And then fewer vote, but generally it is about the same result. But then no party can be blamed for the inconvenience, so I don't think it tells anything.