I live in Chicago. Almost every route has at least 2 airlines (UA/AA). Many have 3 (WN). Some have 4 or 5. Sure maybe the schedule is not exactly the same, and in practice you want to keep to one airline for miles/status but there are very few routes where there is only one direct option and those are mainly international or very small cities. Obviously if you allow one layover then the options explode.

I guess there's are routes (Frankfurt) where technically there are two options but they don't really compete

I don't know the last time I looked for a flight and only had a single airline with routes. There's always better and worse options, but that's to be expected.

no matching literally the same products is distinct from being a monopoly

some specific routes may have de facto monopoly but many, many are not