The Google Squeeze
OTAs have always been a special case when it comes to Aggregation Theory; like Aggregators, they serve customers on a zero marginal cost basis, and they have power over supply (hotels, primarily) by virtue of delivering them demand. The hangup for me is how they acquire that demand: first and foremost from Google.
This arrangement between OTAs and Google has long been beneficial to both sides. Google drives traffic to the OTAs, which can monetize that traffic via commissions extracted from suppliers.2 Google, meanwhile, not only receives relevant results it could serve to customers, but also makes billions of dollars from OTAs buying search ads.