I don't see how a No Referendum will mean chucking out en masse the non-Brits. Those who have been here 5 yrs or so will be eligible to apply for IDR, for instance.
Why bother to get nationalised...
For a start, the hypothetical EU national would be secure in their choice to continue living in Britain.
If I go missing or get in trouble with another country, it's nice that UK will be interested in my plight & be consul/embassy I can contact. Yes I have another nationality, but even better to have 2 interested govts.
As a British national I have a different/better relationship with commonwealth countries (potentially better visa requirements and so on) than as my original nationality.
If Britain Does stay in EU, being a Brit national enables work elsewhere in EU (in case your other nationality leaves EU, not unthinkable for some).
There's an aspect of my job that means I was a stronger candidate for having been naturalised.
==========
Legally the UK can't start doing drastic rule changes about who can vote compared to what they have recently done, people should have been moaning for last 50 yrs about Commonwealth privileges being irrational not just at this moment when it personally affects them.
I shall continue to call myself an immigrant not an expat.