The Rif seems to have thought sugyan b'alma referred to the way the sugya is structured in the Talmud. R. Leonard Levy argues this informed a great deal of his halakhic methodology (see our interview with him here https://mekormayimhayim.substack.com/p/interview-with-rabbi-dr-leonard-levy - his article is linked there, I highly recommend it). But Rashi was probably more correct - it's about how most judges rule once a pattern emerges. Sugyan is first-person plural of a verb roughly meaning 'to go' or 'to walk' - just as in 'halakha'.
As you point out, this does seem to restrict judicial discretion quite a bit (although we are talking about shikul hada'at, so the constraint is less than devar Mishna). In the Yerushalmi, R. Yohanan is understood as holding the less restrictive view that even a devar Mishna is less binding, at least bedi'avad, as I point out here: https://mekormayimhayim.substack.com/p/precedents-all-the-way-down-halakha-c2d.
The Rif seems to have thought sugyan b'alma referred to the way the sugya is structured in the Talmud. R. Leonard Levy argues this informed a great deal of his halakhic methodology (see our interview with him here https://mekormayimhayim.substack.com/p/interview-with-rabbi-dr-leonard-levy - his article is linked there, I highly recommend it). But Rashi was probably more correct - it's about how most judges rule once a pattern emerges. Sugyan is first-person plural of a verb roughly meaning 'to go' or 'to walk' - just as in 'halakha'.
As you point out, this does seem to restrict judicial discretion quite a bit (although we are talking about shikul hada'at, so the constraint is less than devar Mishna). In the Yerushalmi, R. Yohanan is understood as holding the less restrictive view that even a devar Mishna is less binding, at least bedi'avad, as I point out here: https://mekormayimhayim.substack.com/p/precedents-all-the-way-down-halakha-c2d.