Historian Torsten Heinrich at The Diplomat makes some excellent points.

The United States hasn’t produced any conventional submarines since the Barbel-class in the late 1950s; every submarine class since then has been nuclear powered. This might have made sense in the context of the Cold War, where Soviet nuclear-powered ballistic missile submarines had to be shadowed, but times have changed. While previously conventional submarines had to snorkel roughly at least every two days of time under water to recharge their batteries, air-independent propulsion (AIP) has changed the game. German Type 212 submarines can stay under water without snorkeling for up to three weeks, traveling 1,500 miles (2,400 kilometers) or more. Without emitting heat and with no need for constant cooling due to the lack of a nuclear reactor, these German submarines and comparable designs are more than a match for nuclear-powered submarines in terms of stealthiness.

As any US submariner will tell you, the acoustic signature of AIP boats is a significant challenge for US detection capabilities. On top of that, China is building a network of surveillance and detection systems across the expanse of the First Island Chain. Satellites, ASW helicopter bases, and possibly a version of the US SOSUS network, all concentrated in a relatively small portion of the Western Pacific, all increase the chances that US SSNs will find it increasingly difficult to transit and operate undetected and untargeted. Besides, we haven't sufficient SSNs proportional to our reliance upon those assets in the event of a war at sea with the PLA Navy. Conventional submarines are an attractive and sensible alternative to SSNs and could alleviate that situation, asserts Heinrich.

Whereas China can and will create a bigger subsurface fleet than the USN by mixing conventional submarines with nuclear powered ones, the financial burden of matching hull with hull is practically impossible for the United States, at least as long as it limits the USN to SSNs. Conventional submarines might change this. While one Virginia-class submarine costs roughly $2.7 billion per unit, the same money could buy six to seven conventional submarines of the German Type 212 class.

Heinrich makes some other valid points about the potential to keep shipyards employed, and the export potential of such boats. Well worth the read, and serious consideration. I remember well the decommissioning of the last of the diesel boats, right around the time I pinned on Captain. Conventional submarine propulsion technology is light years ahead of the Barbels, built in the late 1950s, and should be explored fully as a means of defeating a capable and growing enemy in the event of a war at sea, especially in the Western Pacific.

H/T: Grandpa Bluewater (who else?)