What specifically, in your mind, makes a submarine obsolete. Because working with underwater systems has shaped for me a different view.
That's because you are talking about upgrading old subs. Whereas the discussion is about purchasing a new class of subs.
Old subs can comfortably continue to operate within the constraints of their design through upgrades. When it comes to new subs, it's all about new materials, new designs, greater performance. We want our new sub to move faster, farther and deeper, carry more etc, which is not all possible with existing designs.
New sub classes, in certain cases, come with the advantage of a higher level of funding than normal. For example, the SMX Ocean was a product of France's nuclear program. So the benefits are passed on to the conventional version, like pumpjet propulsion.
Right now, the navy has the option of either splurging on our future SSK or being more conservative with the SSK purchase because of the SSN program and the urgency to have more new subs. While the other 3 competitors do not have as many options to offer, depending on how it pans out, the French can either choose to participate with the SMX Ocean or a bigger Scorpene. But, rest assured, older designs are not going to make the cut, hence obsolete.