The "antitrust community" is people working in antitrust law/policy/academia who write, tweet, speak at conferences, etc. Two caveats are that a) this is my view based on personal observations, and b) that a consensus doesn't mean that everybody agrees. Undisputedly however, there is a lot of academic work these days which comes to the conclusion that merger review should be strengthened. For instance, see [1].
Khan's aim isn't necessarily to make mergers more difficult, or to prevent them per se. Rather, it's to make a more concerted effort to prevent mergers which on balance, appear not to be in the public interest. There are several reasons why a merger might be harmful.
First, some mergers are "killer acquisitions" whereby a large firm will acquire a "nascent" competitor and then discontinue its product (e.g. [2]). One danger here is that killer acquisitions nip start-up competition in the bud, such that new firms don't have a chance to grow and compete against incumbent firms. That's a problem because competition means that consumers, rather than incumbent firms, ultimately get to choose market outcomes. If there is no choice, then consumers can't choose. The ultimate failure mode here is some kind of command economy where monopoly firms get to make most of the decisions about how markets work and consumers get little say at all.
Second, mergers lead to market concentration because you're taking a market with n firms and moving to a market with n-1 firms. Market concentration isn't inherently bad, indeed, some markets are 'naturally' concentrated. That said unconcentrated markets are generally preferred to concentrated ones because concentrated markets can lead to things like tacit collusion (which has similar outcomes to a cartel) [3].
Third, if there is a real possibility for startups to grow organically and challenge large incumbent firms for the market, then theory goes that VC funding could be stimulated on the promise of a potentially huge return. For instance, a VC might be willing to fund a firm if there's a 5% chance that it will be the next Google, but not if there's a 10% chance that it gets acquired in a year or two.
You make a good point regarding acquisitions, IPOs and the personal incentives to work at a startup. I'm not assuming that IPO's would become more likely or lucrative; I don't have a view on that (except perhaps what I said in the previous paragraph). I guess if a firm was going to be successful, and a merger wasn't an option then it would eventually IPO instead, no? That might lead to delayed compensation, but it shouldn't affect the viability of a solid business. Fewer acquisitions might end up with some firms failing before IPO which would have otherwise been acquired. That's bad from the perspective of those at the firm, but isn't inherently bad from a social perspective. After all competition necessarily entails winners and losers. If all else fails, there is a 'failing firm defence' which would allow an acquisition if the only other option is the firm going out of business [4].
Finally, the lack of enforcement over the past few decades hasn't given regulators much opportunity to "learn" what is a good/bad merger. A more active merger review policy would entail regulators building up expertise and fine-tuning their approach. It's important to remember that merger control isn't necessarily adversarial. At its best, it's a positive-sum dialogue between firms who want to do business and regulators who are trying to provide public-minded oversight.
[1] Kwoka, John. "The structural presumption and the safe harbor in merger review: False positives or unwarranted concerns." Antitrust LJ 81 (2016): 837. https://heinonline.org/HOL/Page?handle=hein.journals/antil81...
[2] https://news.ycombinator.com/item?id=38145568
[3] https://www.tutor2u.net/economics/reference/oligopoly-tacit-...
[4] https://www.concurrences.com/en/dictionary/Failing-firm-defe...