We are where we are - we have a Royal Family, they bring in revenue (yes, alright, they also spend it) to businesses and individuals, it's part of history and sadly, we're a hot spot for a security risk in this country.
The money being spent on security would be spent whether they have a carriage ride or not - but it'd be far more behind the scenes. Protection of the family will of course be first and foremost in terms of what we actually see, but protection of the entire environment surrounding the celebrations, the public, the town, the transport, the area, will happen as a matter of course, whether there was a carriage ride (or any other very public facing 'display' of the celebrations) or not.
£2m is a massive amount of money, of course we could all allocate that to somewhere we hold dear. But no-one is going to say 'no' to the event, howsoever that presents itself (unless there was going to be another Royal 'It's a Knockout'), and therefore the allocated money has to be spent.
She isn't a 'non-entity' or 'irrelevant'. We don't know her, we have no idea as to her persona. As a nation we vote worse people in to represent us!!
Until we have a resurgence of the French or Russian Revolutions (god forbid in terms of the violence and destruction) we are 'stuck' with the Monarchy and there will be thousands of people who will love this event and all its public display. So be it. Personally, I won't be watching, but I'll be happier knowing that the security services will be doing all they can to mitigate the risk to the public.