I think phones can be both waterproof and modular. Going by [1], waterproof gaskets usually come from either glue or silicone. Glue obviously inhibits modularity, but silicone gaskets work fine.
The Galaxy S5 is waterproof, has a rubber gasket over the charging port and a removable battery but not "significantly bulkier" compared to all of the current crop of mid-range/flagship phones - it's actually lighter than the glass-backed ones
Sure, but the reason they're nice to have for that one time that an accident occurs, whether a drink is spilled or you drop it in the sink or whatever.
That's just splash damage, which doesn't really equal waterproofing. There are different IP ratings for the two. Splash damage resistance is significantly cheaper and you'll find plenty of models that don't claim to be waterproof, but are still splash resistant. In other words, IP52/IP53/IP54 instead of IP63/IP64: https://www.dsmt.com/resources/ip-rating-chart/