So all that above to say there isn't a threat to Canada. However, there are direct threats to Canada.
Land, air, sea we are in a fireproof house. That much is very clear. However, we are on the front lines in information, cyber, and space domains. These are where Canadian security really needs to step up and should be considered heavily in any army future structure discussions. There may be a role there for the army (information for sure) that needs to be considered.
The level of tardiness & lack of foresight in Ottawa on defense matters has only been possible because we are lucky enough to have a giant moat on 3 sides of us, and the baddest kid on the block as our neighbour. We couldn't have lucked out more, actually.
I have ZERO knowledge of information/cyber operations, how to conduct them, or how to protect ourselves from them. Technology outpaced me over a decade ago, and the knowledge gap between myself and what technology can do just grows every day. With Russia and China as 'non-friendly' nations with keen interests in our affairs, I do hope that cyber security will be a major focus in Ottawa soon.
Imagine debit cards not working, and ATMs down? Banking history, gone? After all, we don't actually use much cash these days - somebody near a computer could destroy our personal finances in a way that would leave us literally screwed. And when it happens to everybody at the same time? Cell phones won't work? No internet? No power? The cyber threat seems to be like the nuclear threat's little angry brother, and the one we should be a lot more terrified of in terms of probability. As such, a focus on cyber across military and national infrastructure should be a top priority.
My thoughts on planning follows. I'm still learning this theory so some of this may be way off. But this is how I currently understand it:
Contrast that with the Royal Navy. They know what they want (carrier strike, submarine nuclear strike, limited amphibious operations, protection of those assets with the rest of the fleet) and generally how to get there. That's because they have a threat to plan against, and they know the tools to deal with that threat. The threat to the UK is seaborne and they develop elsewhere. The RN needs to be able to strike at those threats before they make it near the UK or her interests.
The UK army is generally developed on capability-based planning (CBP) from what I can see. Which is not ideal. There is no land-based "threat" for them to plan against. So they develop expeditionary forces that are generally balanced with their capabilities. And that means equipment first, doctrine second in a lot of cases. Get the capability and then figure out how to integrate it.
For Canada, we are forced into CBP given that we have no existential threat territorially similar to the UK. So you developed capabilities. So for ships, we want generally purpose frigates because you never know what the job is going to be. And an army that has no focus because again, no idea what the job is going to be. CBP also seems to cause longer-lived equipment. Equipment generally outlives the doctrine because it also outlives the threat. If there is no threat you can often ignore older equipment issues because it still provides a capability. There is not impetus to change equipment because there is no threat.
Contrast that with how we responded in Afghanistan. The UAV's, helicopters, tanks, and RG31's were threat-based planned equipment purchases. They fit into our insurgency doctrine that was developed in response to a specific threat.
Using that as a reference, I would think air assets would be our most valuable in terms of usefulness & versatility. Whether it is NORAD duties such as intercepting foreign aircraft, possibly intercepting incoming ballistic missiles, participating in coalition operations, supporting our friends when needed (Baltic Air Policing, or temporarily taking over for Alaska based F-15C/Ds when the fleet was down, etc.) Plus they can relocate themselves to where needed in the country far faster than any other asset.
Everybody wants to destroy a threat before it gets to their shores. And historically, the RN has been the focal point of that for the UK. The RCN does play an important role in so many ways, but I envision it continuing to deploy as it recently has been - OP Caribbe, Africa, Persian Gulf, SCS, etc. General purpose as we don't know what the operation will bring, but the CSC should be able to handle the higher intensity scenarios a lot better than the Halifax class.
The Army, like you said - also general purpose, as we don't know what the deployment will be or what will be required. Nobody saw Afghanistan happening, or having troops in Iraq, Ukraine, Latvia, etc all at the same time.
Can we be all singing and all dancing, and keep a MBT capability? Yes, we can. We are now. Should we be all singing and dancing though? Personally, I don't know. I've leaned towards 'no'.
Our allies have because their perceived threats are all much closer, geographically. Tanks don't have to go very far if you border Russia, are being invaded by Russia, and are as big as southern Alberta.
Is the number of tanks we could provide to an emergency operation worth the resources of sustaining that capability, when it will take us ages to get them to where they are needed? (Load onto train, train to a port. Find a ship to transport them, or probably 2 ships. Load them up. Sail off. And hope the country where they are needed has port access, otherwise it's back to the train.)
Are they useful? Yes. Would our limited number of tanks be useful in a theater where several other countries have them in useful numbers, especially if ours take a while to get there? I would argue no. The resources we use on that capability may be better used on something else that would be much more versatile for us.
Good points on both sides. Like almost everything else we face as a country/military, a lot of it is waiting to see what the next conflict looks like