You have 2 reasons why t1 would keep slyvan instead of stax. 1.) rbhg performance and 2.) sylvan was signed recently. Both are because stax is igling. If stax chose to not igl. T1 need an igl and need to kick 1 person.
1.) Sylvan rbhg performance vs stax rbhg performance is an irrelevant comparison that you brought up because sylvan here is not igling so his performance isnt hindered, but stax performance is hindered because he is igling.
2.) Sylvan was signed recently because T1 decided stax was igling and he was okay with it. If stax wasnt okay with it, t1 obviously need an igl, which means that 5th spot wouldve been an IGL