Anti-competition law and it's interpretation has changed drastically in the US after Robert Bork's book 'The Antitrust Paradox' (1978). Reagan administration used it as their bible.
Yes. Broadly speaking, the law went from looking at concentration as a bad thing on its own to only looking at whether consumer prices would be impacted. The prevalent idea now is that mega-mergers will reduce prices because of economies of scale.
This is classic example of short term optimization. Sure, prices can be lower now but it might stagnate there for long time because there is no competition allowed. If it is technologically feasible, competition would find the global optimal point eventually in longer term but monopolies will settle in local minima.
Probably why a company like amazon has achieved elusive scrutiny is that they give consumers cheapest prices but squeeze suppliers. Suppliers most definitely hate amazons dominant position.
The law was not changed. The interpretation of the law was changed, as you describe, initially by Republicans but Clinton and Obama maintained the Republican interpretation.
