Everything in life is about incentives, and political alternance between two parties provides absolutely no incentives to do better. You could try and do better to have better chances to win the next election, or you could wait for current events and time to wear off the party and power and have the power fall into your hands, whatever the state of your political option. People will not vote third party because they'll be afraid to lose their vote, and none of the alternance party will fix the voting system to benefit an alternative because, again, there is no incentive to do so.
When I was a kid, I heard pundits talk about how a political alternance was the symbol of a healthy and stable democracy. It took me a few years to understand that stability means that most of the issues people care about are not on the ballot because either both party support the same position, or one of the positions won't be enacted if the party is voted into office.
It doesn't need any conspiracy to happen. It's all a matter of incentive. We have to find a way to either incentivize things to change, or disincentivize things to stay the same, and that's where demagogues will use people's discontent for their own political gain and offer us even more of the same, or even worse. It absolutely won't stop until citizens stop worshipping individuals and organizations over values and principles. If values and principles were our north stars, we would never tolerate that any leader or party betrays these principles, and we wouldn't be distracted by whatever rattle they throw at us to have us betray ourselves also.
The remaining question is : can a population be disciplined enough not to get distracted, and united enough to agree on at least one common goal (however how small) long enough to actually achieve something the people want against a whole system built against the fundamental idea of democracy and national cohesion ?