There's no reason to encourage people onto slower services, as providing capacity on slower services is more expensive because there are more diagrams to maintain frequency. Ideally, everyone travelling from London to Birmingham would use Avanti. Were that the case, LNR could run 4-car north of Northampton, which would free up 350s to provide more capacity by running more 12-cars south of Northampton where the service is busier. Or with the coming of the 730s, it'd be 10 car south of Northampton and 5 north of it.
The only reason to have a cheaper option is to widen the market (i.e. for people who need flexibility and can't afford Avanti fares). But you don't need both Chiltern AND LNR for that, and you can do route High Wycombe for Chiltern. (Though equally it shouldn't be competing with Avanti, it should concentrate on its own markets and incidentally offer cheap fares, which means making the Birmingham services slower).
Bit in bold - which is a pretty crap offer to people from the likes of Gerrards X, Beaconsfield, High Wycombe or Princes Risborough - basically saying 'we're going to slow down your journeys to Birmingham so we don't compete with Avanti' when none of those places are anywhere near a place they could use Avanti and benefit from a quicker journey.