Yes of course I'd be frustrated, and I absolutely agree with you, I'd feel the same way, but this is just the sort of response the RMT are trying to generate, in order for Govia Thameslink to buckle, so they can achieve their aims.
I'd much prefer that option to travelling by train anyway, with the inevitable queuing to get on the Falmer bound trains at Brighton.
The only question is I guess, how far west along the 700 route are you? Anywhere much west of Littlehampton and the journey would take forever.