Antitrust legislation & enforcement are inextricably linked, for all sorts of reasons. This isn't just a US problem.
The entire conventional legal approach was (and still is) flawed. First, monopolies tend to form in The way it works is by "proving" anticompetitive and/or monopolistic behaviour, and fining companies.
Take the recent EU ruling (and pending copycat cases in the US) on the adwords "platform monopoly." The idea was that (1) Google controls the search engine marketing platform and (2) they use this control to favour their own search engine. They proved it. Google got a "cost of doing fine."
First, the whole premise is. If a monopoly exists, we state prosecutors shouldn't need to prove monopolistic actions. Monopolistic practices are proof of a monopoly, but the logical premise behind antitrust law is that monopolies are harmful... and we assume that anti-competitive/monopolistic behaviour happens. Let 3rd parties sue privately for specific abuses, but the state should be focused on preventing monopoly not retroactively punishing use of that monopoly.
If prosecutors have to prove individual abuses separately, they will never make a dent. Most "monopolistic behaviours" are not proveable. They may not even be decisions, they're just what happens in a monopoly.
I totally agree with you about bogus "consumer welfare tests." This is academic hogwash, and it's probably there to provide cover for inaction.
Second, "what to do now" is unavoidably hard, subjective and uncomfortable for a court. Cases vary too much.
From a "greater good perspective," FB could just be shut down. As long as data is destroyed, no one but FB shareholders (and employees) lose. I don't mean that they should be shut down, just that there are no external welfare concerns.
Google is more complicated. Alphabet needs to be restructured into 5-6 companies. Google's (high quality and public benefiting) R&D would be in danger. You'd need to enforce non-collusion between infrastructure, content & ad platforms. It's not something a court can do well, using 19th century case law especially. Case law says nothing about whether or not a ruling was a success.
Mostly, antitrust law should deter monopolies from forming in the first place. Monopolies should voluntarily avoided by companies, because antitrust should be something that doesn't pay to mess with. As things stand, monopoly is the best strategy.
Last, a lot of what needs to happen needs to happen outside of strict antitrust bounds. The phone plans available to you are more closely related to political decisions (eg radio spectrum) than the actions of individual companies. Facebook's monopoly, the most worrying aspects of it, rest on data collection and advertising practices that most people find horrible, and believe should be illegal.
Antitrust may not have even been necessary if (a) those mergers hadn't happened (b) personal data hoarding wasn't a free-for-all and (c) advertising was not so unregulated.
I agree about sleeping on the wheel though.