Niantic doesn't just want data about where we are so that they can sell it to advertisers, they also want us to gather data for them at real-world locations to help develop their Augmented Reality (AR) tech -- that's Niantic's real vision that they might be willing to sacrifice some short-term profit for. One of the things Niantic introduced to the game a year or two ago was AR scanning of Pokestops: getting players to do panoramic scans of the area around a Pokestop. I never do this, so I don't know if you get diminishing returns from scanning a specific Pokestop repeatedly, but there are a lot of Pokestops to be scanned (and they probably want multiple people to scan the same Pokestop as a form of checking for reliability). And to get this data efficiently, they need people to go explore more than they have been doing.
Another thing I would expect Niantic to want to do with the data, although this is more speculative, is observe what kinds of impacts Covid and lockdowns had on communities. This was a gigantic perturbation to the game, and by forcing players en mass to coordinate in real-life again Niantic could potentially study whether old communities reform or how new communities develop.
TrainerTips also claimed that Niantic's position if lots of players abandon the game because of this is that they would put their marketing budget towards attracting new players into the game, which would provide new data about people and their daily routines, how these new players' behavior differs from early adopters, etc., all going into the data grinder they already have set up.