I believe people are arguing for tariffs now are mostly not arguing that tariffs are good but that the world needs to level the playing field and that the US imposing tariffs now is a way to force that to happen. The US has lost quite a bit of industrial production to China and other nations over the years and argument is that this isn't simply low Chinese wages but a variety of trade barriers and trade subsidies. The rosy scenario is that Trump imposes these measures, other nations offer concessions and the barriers are mostly lifted with trade now being more in the US' favor.

The problem with this scenario is that every nation naturally has it's own idea of what is fair and that many things that are trade barriers are around politically sensitive issues (safety and purity standards, etc). So getting other nations to immediately drop stuff isn't that easy.

The analogy is war - after a short engagement, the trade-warring nation will "come home victorious". Like regular war, this scenario often doesn't come to pass.