I don't worry about losing an OC to a HC job if he's good. Because the top offensive minds tend to bring with them and develop coaches under them like the QB coach or passing game coordinator, who can be promoted if the OC gets poached.
Plus, if the OC is good enough to get hired for a HC job, that means Justin Fields likely performed very well. Even if it's just 1 or 2 seasons, Fields' isn't going to crumble and fall apart because his OC is gone. Herbert lost his OC after his rookie year, they hired a defensive coach, changed offenses, and he still performed well. Josh Allen isn't going to fall apart without Daboll. The goal is to make Fields good. The league is full of good offensive minds who can't handle being a HC. Get the best coach, deal with the turnover when it happens.