Yeah, without a way of migrating to Core without having to create a transaction, large holders are incentivised to keep staking through the qt client.

Only way I can see it happening is if there are some other reasons to move to Core. Perhaps masternodes or some other feature of StratisD. I think that just straight up soft forking would be a slap to the face of the long-term stakers, but maybe it's worth it? I don't know personally. I can think of reasons for and against cold staking, so it's not really something that bothers me too much personally. However, having the majority of blocks being produced by StratisD clients does seem to me to be in the best interests of the overall network, so getting people to migrate under the guise of helping activate cold staking seems like one smart method.