If you look from Frost's perspective, the only reason he comes here is purely out of his love for Nebraska. In all other respects, it would be a demotion career wise.
A HC in waiting deal? First of all, what does that even mean? Nothing is written into the contracts that guarantees him the job. It's a handshake agreement, and we all know oral contracts are worth the paper they are written on. If the staff were to fail and the offense struggles under Frost, you still think he would have that HC position waiting for him? Of course not.