For all the "Broncos generate interest in RL in London" mantra being repeated ad nasuem here, you'd think there would be some evidence to back it up.
As any teacher worth their salt will tell you, the enthusiastic students and/or the gifted students will always aspire to the top, to the elite. Without that aim they can drift or choose a less suitable path. I am sure you are not unaware that professional clubs don't run junior teams. There is no Saints under 8s team for example. All the amateur clubs in St Helens run under 8s teams. Saints scout and/or have players recommended to them and they come for trials, maybe are taken on the scholarship programme or their under 16s team (since I think the under 15s was abandoned SL wide when the under 21s were? Or was it the under 16s that was abandoned? One of those age groups was abandoned anyway).
If all a player wants is to play rugby league socially then the local amateur team is fine. However, for those players who aspire to play rugby league professionally, to have no culturally and geographically relevant rugby league club at the elite level to aspire to will be detrimental both to those players and the sport as a whole. And it's no good saying that northern teams will pick them up. I know that the Sussex Merlins, for example, have been trying to contact northern Superleague clubs for weeks now as they have some interesting players but they haven't even had a return email. Northern clubs, sadly, do not want to know, not least because they have lots of available talent in the north but also because they are invariably too blinkered to consider that actually there maybe southern players who are more talented than their northern counterparts and sending a reply email may actually be worthwhile.
If there was another Superleague club in the south of England then losing the Broncos wouldn't be so much of a problem (although obviously the more, the merrier). But them being the only elite level club in a region containing millions of people means that if they are lost then I think the gap will be too great and potential will be lost to the sport.
There is also, of course, another angle to this: we struggle for England players in certain positions. How do any of us know that a brilliant England centre isn't at present languishing down south because there is no way of raising his profile?