But honestly - who in their right mind would apply for the outrageously expensive, cumbersome UK fiancé/spousal path if they qualify for the EEA route? Brexit or no Brexit. No need to worry about "what ifs" until we know what we are dealing with. I imagine they could switch paths down the road if they were concerned.
I think the opposite
that you would have to be a risk taker to use the ever changing EU rules when you could sponsor on an UK "visa".
EU law will end on a Brexit and they need at least 5 years of EU laws. I would choose the safe option of a UK family visa and begger the cost of those visas.
In addition to Brexit, the OP would only need to lose his job and struggle to find another and they are then in all sorts of problems with free movement. If he stops being a
qualified person, then his spouse has lose his
"right to reside" in the UK.
EEA citizens and their families can lose their
right to reside at any time. RCs becomes invalid, regardless of what the end date says. An RC is given in expection that the EEA citizen will contiue to be a qualified person at all times.
Those on UK "visas" only have to meet the requirements when they apply for another visa. Their visa is good to go, right up to the end date. On the second visa, the visa holders earnings can be used to meet the requirements for the next visa.