Because it's a reduction in capacity and frequency to London
No it isn't. In the high peak, when capacity is most in demand (arriving London Bridge or Blackfriars 0800-0859, when demand is highest) from Brighton is:
One service from Brighton to London Bridge
Two services from Brighton via Blackfriars/Thameslink.
So May 2018 high peak service level Brighton-London will be exactly the same as today...
Rest of the day, I've never even come close to strughling to sit down on the 4tph service. Nobody loses frequency *to a particular place in London* (few people just travel to "London" generically, London Bridge, Victoria and Thameslink are distinct markets) - in fact everybody gains (except for peak travellers, who stay the same), and there is easily sufficient capacity for off peak passengers with 3tph (all 12-car), which will only be for 6 months anyway until 4tph comes in.
And all core-bound journey times get faster too.
Yes it is a reduction of 1tph in total service to "London" *off-peak*. But a difference very few passengers will actually notice or care about or have any demonstrable impact on their journey.