I don't know how much resistance that needs, but I'd also really like it to withstand the occasional drop in a pond.
If a plain rubber seal does that, fantastic. Though I'd really like to be 100% certain that I've closed things sufficiently.
That's hundreds of dollars a year in savings. For me it would be about $330/yr in savings, enough to cover my streaming video and music bills completely. That savings falls in half if my time is valuable and I've got to drive to an Apple store and pay $90 for the replacement, so I've never done that. Instead, I go on Amazon.com and order an entirely new phone when my battery gets to be an issue.
I think replaceable batteries will make calculus work for me and billions of others over the long haul. Maybe 50% of consumers take advantage and save money, 49.999% never notice or care, and 0.001% miss scuba diving raw-dog with their smartphones and have to buy a specialized case for piece of mind.
I suppose that there will be some number of phone replacements delayed if battery replacement were cheaper and easier. But I'd bet it's a relatively narrow range between "willing to let a pro do it" and "might as well upgrade the features anyway".