Channel factories will be able to open and close many channels at once, so if you open 100 channels in a single transaction, well then you can divide the onchain cost by 100.
All of this also completely ignores the LN routing problem which is conveniently left out of the LN whitepaper as it is actually unsolvable at large scale unless you have very few supernodes coordinating everything. In the real world those are commonly known as "banks".