Well you ask a reasonable question Beaufort so I'l try and answer it. The fact is that not all six Tris are available at any given time. One of the main reasons for replacing them is the amount of maintenance now required to keep them flying. Certainly you're 'losing' six aircraft and replacing them by three. That is a simple fact BUT there will still be a 'spare' aircraft once the third Dornier is sourced and in the interim the LCY aircraft will be available, although not on 'immediate readiness'.
I hope that you can see that Alderney is gaining more seats. As I said, no airline today can afford to have spare aircraft just sitting around 'in case'. That leads to bankruptcy then you have no aircraft on the routes. There might well be small charter outfits who offer flights to Alderney but running a scheduled service is something completely different... I know, I've been involved with both types of operation. Finally, there is absolutely no guarantee that a new carrier would be able to get a slot at SOU, and the ACI-SOU route really is a lifeline. Other routes have been tried but they have all been dropped due to lack of support. No company can afford to have aircraft just sitting around or flying half-full, unless of course the ticket price is based on a very low seat occupancy rate, which inevitably means very expensive tickets.
I do know Alderney quite well, I was based there for a while with Air Sarnia and transited through with Aurigny. whilst I would and have gone back for a visit, one can 'do' the island in a very short time, and that is what non-aviating but sailing friends have also told me. Off-topic so apologies, but most yotties are just passing and have a limited cruising time, so whilst Alderney is a pleasant place to visit for perhaps a day and a night, other pleasures beckon. I know all about the 'Alderney roll' and believe me, it's not just when the wind is out of the NE! A marina might tempt more to visit but it's a financial gamble and I really don't think it would pay off. I'm sure we would agree there are other things that the money could be better spent on...improving the runway for one thing.