I'm not sure I'd characterize it as crossing a picket line but I hear what you're saying Bonway. There are (as there always are) many sides to this.
SAA has just transferred the last B737-800s to Mango and now there are potentially 60 odd SAA pilots who could face losing their jobs. Therefor not everyone can afford to take the same viewpoint as you and need to put bread on the table.
One of the reasons SAA has taken that decision is because the pay and benefits at Mango are so much lower than SAA. Safair is even worse. And this isn't a case of SAA paying too much, it's that Mango and Safair pay way below the market rate for pilots.
So if a Mango pilot or a Safair pilot took up a Ryanair contract they'd they'd actually be standing in solidarity with their SAA, Mango and Safair counterparts by showing their own management that they are undervaluing and underpaying them.