Ex-Dragoon, I feel a bit out of my lane to answer the question, :-[ but I'll say the following:
From what I've read, yes, submarines are required to diversify our fleet. I think that diversity is one of the best tools to approach, analyse and solve a problem if all the diverse components are well coordinated, and the complementarity is optimized. That is not an easy task !
Submarines look like a good functional complement to the fleet, it gives a three dimensional view and capability to the fleet, I mean boats sail along the surface that gives you the two dimensional capability and forces, OK, I know that warships can launch rockets and torpedoes but still submarines add the third dimension down as much as aircrafts added the third up.
Now, a bit out topic but we will have soon a third coast which is the North that is going to have a huge geopolitical impact. Everyone knows that people want to sail through it. Also a lot of gas, oil and so on could be found there. Many submarines are going to be there, I don't want to start a paranoia crisis, but I'm not sure that those submarines are going to be friendly to us. So having submarines will help us to secure our north sea? I think so but here I am totally out of my lane !
Many examples can be found about how efficient and deadly submarines can be. During the WWII, isn't true that some German submarines made up to St-Laurence river? Are submarines only efficient during cold war or world war types? I don't think so. Look at North Korea, they have high number of submarines in proportion to their fleet, why is that? What is in the back of their heads?
From the links I have put, I was concerned about countries of our size (population and economical) having a fleet that seems better equip than ours. Don't get me wrong, I am not talking about the quality of our NAVY where I am convinced that all members work hard and they are first class soldier, I am just talking about money thrown in equipments.
So like I said, I would like that our NAVY being competitive on the water as much as under the water. I would like that our eyes and ears are first class sharps on the surface as well as below it and that we can take action in the 3-dimensional maritime space. Thus I believe that submarines are mandatory to secure sailors on the surface, to occupy the sea space and to keep our fleet competitive.
P.S. I have edit the present post to clarify some of my sentences.