You’re making an assumption that the feedback loops are all well understood. They might be, or maybe there will be some runaway effect, some source of carbon or other greenhouse gas that’s completely unknown, gets released, and boils the oceans.
If we manage to keep the warming to levels seen in previous warming periods, humanity might come out better on average in the long run, but the planet is heating faster than it did in those other periods and we haven’t demonstrated any ability to control ourselves. We’d have to stop generating CO2 pretty soon to avoid surpassing the last great warming period.
Some haploid forms are full grown adults, such as male ants and bees. The haploid form of fungi is usually dominant over the diploid form. Haploid forms are not some crippled half version. They are a complete life stage of the organism.
The fact that our diploid forms are what we call people, while the haploid forms are microscopic probably has more to do with how we have sex than anything.
You might argue that the diploid form is more robust genetically, but the sperm cell has great need for robustness.