For the sake of argument, lets assume that somehow (quite unlikely)

Why is it unlikely? Do you believe that the cost of electricity cannot be higher than expected mining revenue?

Or do you expect miners to keep mining when it costs them money?

half the mining equipment gets shut off.
The amount of hashes/second is such that it is currently, lets just say, quite
secure against any takeover.

The equipment won't be simply turned off, it will be up for grabs.

Please check this web sites:

https://nicehash.com/

https://www.multipool.us/

One can use them in the same way he uses normal mining pools, and they switch between different chains.

Say, multipool.us can switch between BTC and PPC (Peercoin).

Mining BTC will be less profitable after a halving, so a miner who is willing to maximize his profits might use multipool to auto-switch to something more profitable.

Which might be attack-on-Bitcoin.

E.g. if 60% of bitcoin's total hashrate is available via "multipools", one can try to pull of a double-spending attack.

Your document makes a long series of assumptions about how this can turn out
bad with each individually is implausible, together are just fiction.

It sounds like you failed to grasp even basics.