I would think more in "game setting" and not in "reality standards" - there are quite strategic aspects when homeless units may still be active (at least for a while).
On one hand, maybe sometimes you have to think about first, whether and when especially KI countries with a great navy should be conquered completely or rather should offered peace - and/or let the last province/s for another avid conqueror.
(because without any land there is no government which would agree to peace as you know now; and if that nation has a lot of subs for example the remaining playing time can be very funny)
On the other hand it's currently so, that ground forces which have no homeland are still able to conquer new lands.
But all the same, in my view that should not be possible for an endless time, and I think the best solution would be:
Units without any homeland should lose morale, in a reverse manner as in healing, day after day a little until they are destroyed...
Then they can cause trouble only a limited time...
And in that time you could still hunt them if you want...
For my opinion suggestions as "surrender to the victor" or "integrate to an allied country" are not really good ideas.
Who wants to take over and feed those units after each war, mainly marine units probably, which most likely may not fit into the own strategic and economic plan.
Then it could easily happen that a players economy will be "Ruined by winning wars".

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