Kirkhill, you are looking at flexibility from a Navy overall capabilities point of view, not from an individual ship or class of ship perspective. This thread is about a single class of ships, the combatant ones, not Navies as a whole, and it is this limited perspective I have always used as guiding my posts.
The overall flexibility in a Navy from the combination of all of its ship's type is a strategic decision. In Canada's case, it is not something derived from the Navy's choice alone, but from NDHQ. If NDHQ determined that having amphibious capability was required of Canada, the Navy would propose what it needs and the "powers that be" would then decide the extent to which it will provide the Navy's request for materiel to do so. Then everybody would get on with the job with what is provided.
Again, though, it is important not to confuse overall flexibility with national requirements. The Australian and the Dutch have amphibious capabilities? Yes, but they live in an area where their own territory has multiple other nations nearby where they may need the capabilities (we are talking a few hundred nautical miles here, particularly for Australia - the Dutch have oversea territories as well where the capability may be needed).
For us in Canada, on the other hand, we have no territory nearby that threaten us from a military point of view (I will except St-Pierre et Miquelon for obvious reasons). The only "military" need for amphibious ships would be for [very] far oversea employment. This requires rather a deployment model based on the US Marines style of permanent forward deployment of soldiers onboard large ships for long period of time. This would be too much of a drag on scarce defence dollars both from the Naval and the Army point of view (can you see what would be required in Army numbers just to make it possible to maintain 1,500 soldiers battle groups deployed permanently in 6 months rotations at sea?). If not for that type of deployment, why spend 100's of millions of dollars every year just to make it possible for the Winnipeg Riffles to practice landing on the North shore of lake Superior once in a while for a change of scenery?
Also, you seem to have an unnatural fixation with crew reduction. First of all the Danish OPV's you are talking about do not have a smaller crew than the MCDV's. When they deploy in their ordinary role (coast guard/constabulary), they have a crew of 18, but only carry two .50 cal MG's. If I wanted to deploy the MCDV in a similar role, I could actually better that and sail with a crew of 15. When the Knut Rasmunssen class deploy with more advanced weapons and a military mission, the crew goes up to 43.
But, and I can excuse you here as you have no experience of working with European (unionized or near unionized) navies, you have to consider concept of operation. The continental European navies, unlike us, the Americans and the other Anglo-sphere navies, do not permanently operate AS IF AT WAR, meaning that we have at least 1/3 of the crew manning all the fighting stations at all time, and often more than that. For instance, if they are out for an ASW exercise, it will be scheduled to start in the morning, end in the afternoon, and after its conclusion and until the next exercise the morning after, the ship will just sail around with a minimal crewing of a few seaman per watch akin to a merchant ship. Some of those nations boast their capability to operate with a duty watch of four: An OOW, a seaman, one engineer (at a remote control console on the bridge) and one signalman. That is fine until a passing merchant ship decides to turn and ram you (which you didn't see coming because the ops room is down for the night and can't quickly escape from when you notice because your sole engineer can only operate the cruise diesel but not flash up the two gas turbines in the engine room) while everybody is asleep. It's a lesson the US learned at Pearl Harbour, and we emulate in large part.