All of the big companies know that most people aren't willing to base their purchase on whether or not a phone has a headphone jack, and therefore they don't have much to lose from ditching it. They can use that space for more battery or for whatever else they want to put there. The inside of a modern smartphone is a pretty space constrained place.

Buying 6 dongles would also only cost $54 ($9 each) which isn't a ridiculous price for an accessory in my opinion, especially if you're buying a high end phone that's going to cost you $700+ anyways.

Personally, I just carry one dongle attached to my keychain and keep one in the car. It hasn't been an issue for me yet.

The space argument makes sense in theory, but one very dedicated person added a headphone jack back to his iPhone 7, and it was possible only because there was a suspiciously empty space where the jack used to be:

https://strangeparts.com/bringing-back-the-iphone-headphone-...

I'm also not persuaded by the "most people won't base their purchase" argument, as that applies just as well to adding another 5-10% to the battery. Most people don't base their purchase solely on anything. I may have to go by my local phone stores and chat with the staff to see how much this matters in practice to people.

It makes IP68/69 waterproofing standard difficult to achieve. I suspect that is the main driver.

I don't think it does. It just exposes three (4?) contacts. Less than a USB port. There have been plenty of waterproof phones with jacks.

Galaxy S7 and above are IP68 rated and still have a headphone jack. iPhone X is still IP67 w/o a headphone jack.