In regards to amphibious force it is absolute necessary for Canada to have one. Not so much from kinetic perspective (although the need is still there), but as a HADR/ aid to civil power it is a must have need. For example west coast is just waiting for the "big one" earthquake to occur. Yet the west coast is isolated in terms of road access (one big earthquake and the Rocky Mountain road passes are shut-down), while Vancouver International Airport is right on the coast (will likely be shut-down during a massive quake/ after effects), and all active military units that might be able to respond on the west coast are based on Vancouver Island; with no meaningful air lift and absolute no organic sea lift. If only for this possible HADR response on the west-coast Canada needs an amphibious force imho.
I'll just speak to the Vancouver island and adjacent British Columbia coast natural hazards issue. There have been some significant earthquakes generated in the region, including Alaska, that have caused significant damage on shore but also have generated destructive tsunami. The last significant one was the 1964 Alaskan quake.
A natural hazard is an unwanted negative natural event system interaction with a human use system. Assessing how these natural hazards impact upon human use systems can sometimes be quite problematic especially if the return period (how often and the time period between events), magnitudes and impacts etc., are unknown because of a lack of data. We have to look at a history of many thousands of years, preferably many tens and / or hundreds of thousands of years, trying to find a pattern. That data we obtain mostly from the geologic record and latterly from the human historical record. With that data it is then possible to assess what natural hazards are likely to impact a given geographical area. The reliability of the assessment is dependent upon the quality and quantity of the data obtained.
The western coasts of Canada, plus the US states of Oregon, Washington and Alaska are all subject to three major geologic hazards because of their location on the Pacific Ring of Fire. These hazards are earthquakes, volcanism and tsunami. The earthquakes and volcanism are a direct result of the plate boundary between the Pacific tectonic plate and the North American tectonic plates. The tsunami hazard is from tsunami generated locally by earthquakes, volcanism, submarine landslides or far field generated tsunami, e.g., Japan, Russia etc.
Any one of those three hazards has the capability of severely disabling or destroying infrastructure, as us Kiwis are only to well aware. If the Vancouver ports, airports, road and rail infrastructure were severely compromised or destroyed because of a disaster, the only way to get aid in and refugees out maybe across the beach, the same as Kaikoura last November. The Canadian Govt could justify and sell an amphibious capability to the Canadian public on the HADR aspect quite well. It could be a politically astute move.