First - airlines, pharma, and financial services are nowhere near 'oligarchy' and don't come close to meeting the definition, just the opposite, they are very competitive.
With Telcos - the issue is real, but the drawback is mostly competitive innovation and price. Stagnant deployments and high prices would be the result. FYI - the US has decent broadband pricing. See: Canada!
Oil is a special one, but the industry is global and very competitive, especially in certain layers of that industry. Prices are widely known and understood. There is zero concern that 'some big entity' will control all of the oil, certainly not in America.
FB, Amazon, Google represent far more existentially problematic kinds of 'monopolies' because it bleeds into other aspects of life (social, media, information) and into other, adjacent industries. These are the real problem.
Search and social also have 'natural monopoly' kind of conditions which exacerbates the problems.
The other industry you didn't mention, wherein there is an existential problem with competition is 'health insurance'.