If Brexit happens, whether we Brits living in the EU continue to have the right to remain resident in EU countries will depend very much on exit negotiations. Given the UK Brexiters claim they want to stop EU citizens living in the UK, then, if they got their way, I think it’d be very likely EU countries will tit for tat. Thus either Brexiters will be proven liars, or we expats will have problems living our lives. If they are proven liars, then they’re unlikely to do well in the Brexit negotiations with the rest of the EU, because they won’t be trusted to keep any bargain they try and make, so the remaining UK will end up with a bad deal, which will screw us expats too. All that’s why I have little confidence the out negotiations will go well to us.
For me, the solution is to accept that if the UK wants to walk away from us Brits living in Europe and sulk like a spoilt toddler in the isolationist corner, whining about having to share its toys, then let it sulk and whine, & give up on it. I’ll take another nationality. Nationality isn’t some magical bean of belonging, it’s a necessity of life. It has many providers, and if one provider fucks up, find another. Mind you, that takes work. That’s why I’m learning Luxembourgish.