That's an imagined and unrealistic scenario. Most countries with proportional systems have a threshold that requires something like 5% of the vote to earn seat

That's an imagined and unrealistic scenario. Most countries with proportional systems have a threshold that requires something like 5% of the vote to earn seats. So nobody with 2% of the vote is going to be part of the coalition. Furthermore, pulling out of coalition, without a very good reason, is an excellent way to ensure that nobody wants you as a coalition partner in the future. Yes, some countries with PR have government stability issues. So does the US, see government shutdowns, accusation of obstructionism, etc. That's just an inevitable consequence of humans disagreeing about policy.