I think the most important thing to remember is (regarding Mercedes staff going to work for Red Bull), is that a good number of the Brackley team have experience working as Honda. So it really does seem to be the best logical step for Red Bull, to try their hardest to acquire that particular knowledge and expertise.
Possibly even worse if Brackley GP just tapers off and can't compete with Red Bull.
Personally i can't see Mercedes tapering off a such, to the point of not being able to compete with Red Bull. But i do think Red Bull will lessen the gap even more than they currently are.
The boys and girls at Mercedes AMG F1 in Brackley, are an extremely talented and highly skilled team of people (arguably the best), but with Red Bull's new acquisitions, they probably won't be far behind them.
[Interesting fact about Honda]:
On my tour of the Mercedes F1 team's factory in Brackley (a few years ago now), i learnt that when the team was operating as Honda, Honda would only allow one dedicated expert (from Honda Japan), to run the engine rigs. Obviously things have changed a lot since back in the Honda days at Brackley, but i found it really cool learning about the Mercedes teams past, operating as Honda and Brawn GP.