I think this polygon point (forgive the pun) is really touching on the point, with a larger work area pre-rendering can take place, not just in graphics but also music, but with a larger work are, there is more room for routines...
You could argue that the Cart would / could provide all thats needed, and indeed this is true for the most part, but with the more needed the more memory hence more expensive.
Its a huge shame the MegaSeedy never really took off, but Arguably it did show some of the limitations of 64K RAM even if the MegaCD had its own cache.
In the long run, with good programming structure, (i.e. you know you only have 64K work area) you can Pull routines in and out of that area, but this came with time as developers learned to make full use of the platform.
so in a nut shell, yes the Cart system with 64K workspace (for sound / graphics / game routines) will work with good programming but its not the better solution as time / history has shown us.