"Think about it, though: if the problem was as endemic as is implied,"
It is, and most of your questions have obvious answers.
"why don't other vendors come up with alternatives?"
They do. Microsoft's Surfaces use a magnetic charger for the same reasons. They're not the only ones who have done so. The primary reasons that companies choose not to do it are backward compatibility (E.G. most Dell business laptops used the same size barrel adapter before they adopted USB-C) or patent fears (Apple's sued people before for similar connectors and everyone was expecting them to do the same to Microsoft but it didn't happen).
"Why hasn't Apple put it on their phones?"
Two reasons. First, phones aren't usually as likely to have the problem. The reason is that phones have batteries which generally last longer, so people are rarely seen at a table, working on the phone, with the phone plugged in. Laptops do have that more often, so they need the protection more. Second, phones are already expected to fall more often, which is why people often put their phones in cases. Because they're small, easily dropped, etc. they've been designed for abrupt falls more than laptops have. It's usually less of a problem if a phone falls to the floor than if a laptop does so.
"(I previously mentioned my LG phone with a magnetic USB adapter)."
Which is useful, as is wireless charging, because they reduce wear on the charging port. Magnetic USB-C cables do exist, and I need to buy one to see if it can withstand long-term high-voltage charging. I'm afraid that they're not designed for that and will fail in a laptop where they wouldn't in a phone.
"Why don't the business laptop vendors (Dell, Lenovo, HPE, etc) address the issue you describe by making the power socket easily replaceable?"
Why don't they make the hard drives easily replaceable? Or memory? Or WiFi cards? The usual answers are that they can save themselves money, produce a thinner machine, and so on. Also, few of their customers are planning to resolder a power port anyway, so they probably figure they don't have to worry about the issue costing them customers. If it leads to a faster replacement cycle for the laptops, I doubt they'll complain.