Planners say the country will likely need bigger, quieter boats that can perform stealth missions, launch undersea robots and fire guided missiles at shore targets.
A nine-page briefing note for the country's top military commander last year sets out the justification for keeping a submarine capability, and comes at a time when the Harper government wants budget savings in both the near- and long-term.
The report looks at what kinds of boats will be on the market between 2020 and 2050.
Ottawa was awash in rumours last spring that the current submarine program was on the chopping block because of its enormous expense and repeated setbacks, including a fatal fire aboard one boat in 2004.
"Submarines are the ultimate stealth platform, able to operate in areas where sea and air control is not assured, and to gain access to areas denied to other forces," said a May 9, 2011, briefing for Chief of Defence Staff Walt Natynczyk.
"A capable submarine force creates uncertainty; countering them is difficult, expensive and cannot be guaranteed."
Investing in submarines is prudent because "in the event of global tensions these relatively cheap assets will counter projection of power and hinder freedom of movement and action."
According to defence experts, that was a veiled reference to Arctic sovereignty, which the Harper government has made key policy platform.
Dan Middlemiss, who has written extensively on naval strategy, said the government clearly sent a message to the navy last year about the current, troubled fleet, warning: "Get these boats in the water or else."
That the program has been in jeopardy was subtly underscored by the navy's top commander, Vice-Admiral Paul Maddison, during an appearance before a Senate committee earlier this year.
"Assuming that Canadians will continue to see a submarine capability as a critical capability for our Canadian Forces," he said, "I would envision initiating a next-generation submarine discussion within the next three or four years ... to ensure there is no gap in submarine capability, which is what we faced in the 1990s."
Under questioning, he was more pointed: "In terms of surveillance of our ocean approaches and the protection of our own sovereignty, I would consider a submarine capability critical.
"And so to lose that for a G8 nation, a NATO country like Canada, a country that continues to lead internationally, and aspires to lead more, I would consider that a critical loss."
Maddison told the committee he didn't envision replacing the four existing boats, purchased from the Royal Navy in 1998, until the late 2020s.
But it may come sooner than that.
Much hinges on whether engineers can successfully extend the service lives of HMCS Victoria, Windsor, Corner Brook and Chicoutimi, which are already nearly 20 years old.
That assessment, including affordability, is still underway, said Middlemiss, who taught at Dalhousie University Centre for Foreign Policy Studies in Halifax.
"I think there is Sword of Damocles over the heads of submarines at the moment and I know the navy, and I think the dockyard and everybody, have had this brought home repeatedly and vigorously and are now trying to play catchup," said Middlemiss.
Other naval projects, such as supply ships and Arctic patrol vessels, have been postponed until later in the decade, and "I think most expect more cuts and outright deferral to come," he added.
The Harper government is currently in the process of rewriting its marquee defence paper and a spokesman for Associate Defence Minister Bernard Valcourt wouldn't say what the future might hold.
"Our government is investing to ensure we have the right mix of naval capabilities to protect Canadian sovereignty," said Chris McCluskey in an email.
"There is no plan to replace the diesel-electric fleet purchased by the Liberals."
The briefing note points out that the market for submarines, especially emerging powers such as India and China, has grown by leaps and bounds, but there are still only a handful of countries in the world capable of building them.
At the top of the list is Howaldtswerke Deutsche Werft, which is a division of Thyssenkrupp Marine.
The company's senior executives were part of a German trade delegation that accompanied Chancellor Angela Merkel to Ottawa and Halifax last week.
Middlemiss said giving up the capability would potentially leave Canada blind in the Arctic because nations are required to notify each other when their submarines are operating around the territorial waters of others.
"The current subs are, despite the bad press, incredibly useful and will still be of equal or even more value as climate change wrecks havoc in the Arctic," he said.
The briefing note said that the traditional Second World War perception and use of submarines has been refined.
Where once they were torpedoing enemy shipping, subs are now more useful in coastline surveillance and intelligence-gathering, as well as being able to launch guided missiles at shore targets, the way U.S. and British boats did during the Libya campaign.