Suppose someone would invest billions in obtaining a mining rig that single handedly achieves 20%, or 30% or whatever% of the mining power of the network, then

Suppose someone would invest billions in obtaining a mining rig that single handedly achieves 20%, or 30% or whatever% of the mining power of the network, then they effectively use that advantage to achieve manipulation of the network or manipulation of the economy via their monopoly/large-entity level oversight of the economy. As you point out, there is no incentive to kill the network, but quite a bit of incentive to manipulate the network. Via modern corporate shells, no doubt people/groups are already trying to achieve this, they do in any market.