1. SH is/wants to be a PK crew.
2. PK crew must be > Emperor crew.
3. With the current SH, they're still relative to other Emperor crew.
4. So a real powerhouse is needed, especially :
4.1. Luffy wants to make a crew to surpass Shanks' (Chapter 1 promise)
4.2. SH will fight against the WG
4.3. SH may or may not need to surpass Rocks in legend, fame, and/or power.