The real kicker on the West Coastway is that there is no reasonable alternative. Between the lack of coach service, the poor train connections at Clapham, and the slow, now interrupted service by train along the coast, this corridor is poorly served. Just in time for the Easter and Summer holidays too! I look forward to packed, sweaty 377s.
As someone who regularly travels for work and leisure between businesses and friends along the coast (Sussex through to Dorset), these changes make the train a much harder choice. The changes don't particularly increase overall timings, but they remove some good (if tight) connections, and frankly the thought of yet another change being forced upon you at Chichester is leavened only by the half-decent café. Hopefully it'll be open for those evening connections: I'm sure the civil servants have thought of that, right?
Still, it's better to know in advance that you have a rubbish connection at Southampton and will be kicked off the train by Barnham; it makes a change from the penny lottery that has been going on up to now.
I suspect there is a reasonable business to be made here from a canny coach / express bus operator. It's not going to be the easiest corridor to run, as the A27 is a challenging coach corridor traffic-wise at the best of times, even before you start the Arundel works. But there is a market there - students commuting along the coast, plenty of day trippers to Dorset, Gunwharf, and the west of England, and also a significant business market. Someone needs to put the Department for Transport under some pressure here: it's civil servants making these decisions, and they can make these decisions at the moment because the revenue on these flows (limited as it is) doesn't have an acceptable non-rail alternative. People will argue that this market might switch to the car, but, again, I wouldn't wish an evening on the A/M27 on anyone, and that's if you have a licence/car/budget. If the service is truthfully more resilient as a result of this, then that might make it a bit easier to swallow. But nothing about this screams resilience: it looks more like a continued lack of staff, and no desire from the DfT to subsidise the service. As you say, pompeyfan: managed decline.