Current AIs are not able to “merge” with each other.

AI models are routinely merged by direct weight manipulation today. Beyond that, two models can be "merged" by training a new model using combined compute, algorithms, data, and fine-tuning.

As a result, humans don’t need to solve the problem of “What if a set of AIs form a unified coalition because they can flawlessly coordinate?” since that problem won’t happen while humans are still in charge. We can leave this problem to be solved by our smarter descendants.

How do you know a solution to this problem exists? What if there is no such solution once we hand over control to AIs, i.e., the only solution is to keep humans in charge (e.g. by pausing AI) until we figure out a safer path forward? As the last sentence you say "However, it’s perhaps significantly more likely in the very long-run." well what can we do today to reduce this long-run risk (aside from pausing AI which you're presumably not supporting)?

That said, it seems the probability of a catastrophic AI takeover in humanity’s relative near-term future (say, the next 50 years) is low (maybe 10% chance of happening).

Others already questioned you on this, but the fact you didn't think to mention whether this is 50 calendar years or 50 subjective years is also a big sticking point for me.