Here's a short article that raises an alarm because of the increasing number of submarines under construction around the world.  A mention of how non-nuclear sub effectiveness may be increasing with air independent propulsion and newer weapons.

  American Thinker article...

AIP Technology Creates a New Undersea Threat
by Edward C. Whitman

As interest mounts in "Air-Independent Propulsion" (AIP) for enhancing the performance of small, defensive submarines, a serious new underwater threat is developing in littoral waters. Increasingly, smaller nations unwilling or unable to accept the high cost of nuclear power to achieve greater underwater endurance and longer range are turning to lower-priced and less ambitious alternatives that still offer significant operational advantages over conventional diesel-electric submarines.