People already complain about the price of that thing. Include a better SoC and it's going to get more expensive.

The BoM cost of the i.MX 8M Quad in the Librem 5 is $35.49. The BoM cost of the Snapdragon 865+, when it was brand new, was $57. That $22 price difference brings a 700% performance increase.

Will mainline Linux work on Snapdragon 865+?

Do you mean that it heats up too much? I don't think this is accurate after latest updates. Also, suspend is not implemented yet. It will greatly increase the battery life and make it colder.

"Hot garbage," in this case, is just a colloquialism that means "it stinks." Garbage smells; hot garbage smells worse. They're not literally referring to heat or temperature.

It's the most modern SoC that supports mainline Linux with FLOSS drivers.