I think the opposition to the merger/takeover (however you might characterize it) is more politics than economics. The global steel industry has been subject of a lot of acquisitions, mergers, and sales. Ultimately, the largest threat to US steelmakers has been China, but the various tariffs on steel imposed by the Bush II, Trump, and Biden administrations have fended that off to a large degree, possibly at the expense of the consumers or the larger US economy.
Locally in the Pittsburgh area, opposition does not seem as strong as some reports suggest, particularly among local US steelworkers who have legitimate concerns that the modernization of local US Steel plants could be jeopardized by rejection of Nippon's offer.
The US steel industry saw some substantial takeovers by India's Mittal Steel earlier, but those were reversed when Mittal sold those holdings to Cleveland Cliffs, a US-based company. The major Russian steel company, Severstal, had significant US holdings in this century, but those were sold off as well. The world iron and steel trade is a very complicated marketplace, influenced not only by economics, but also politics and concerns of national interests and security.
I think the name US Steel plays into reactions to the proposed merger because it sounds like a takeover of an industry, not a single company to people who know relatively little about it.
Big Al