False dilemma.
They are proper nouns.
Beacon chain is simply the name of the network launched a bit over a year ago. It is likely intended to be named as an aspirational thing to move toward.
Mainnet is the network launched 7 years ago. Usually distinct from "test networks", testnet, and seen far beyond ethereum into all cryptocurrency networks, and inherited from the idea of test, development, staging and production environments seen in the best practices of all web projects in the broader tech sector.
This is an article about them merging.
What is "weird pretentious verbiage" about that? If there was anything odd it would be "the docking" and that article immediately says that was a former name, making it a moot point.
The chain names (e.g. "Beacon") are basically version codenames - think of them like the "Buster" in "Debian Buster".
"The Merge" is the (arbitrary) name they gave to a specific future event, because people like to be able to easily refer to things they're talking about. I assume the previous name was discarded due to political correctness.
Curious but it's hard to Google this. What was it?
very much a global tech startup!