From the start the HH was compatible with 40k, we have no data on how it will impact sales by not being compatible. All BoC and BoP was make the point of entry easier, lots of people like me couldn't afford or were unwilling to pay FW prices for basic troops.
In my mind being compatible is very important as most people play 40k before starting 30k. It is hard to start a HH community when you need different base rules and have no one to play with until other people transition, with compatible rules you have far more options for who to play. of course it is ideal to play 30k vs 30k, but that isn't the reality for many.