GEOS 2.0 was in fact released for the C128 in 1989. It did not spur continued interest in the machine, similar to how GEOS 1.x was not a primary driver for interest in the C64.

GEOS was also released for the Apple II and PC/GEOS for IBM PC compatibles in 1988 and 1990 respectively, even beating Windows 3.0 to the market. Neither version established any substantial market share.

There simply was never substantial demand for GEOS. While actual sales figures are hard to come by, it seems very likely there was substantially less interest in GEOS than in the Amiga platform. It would not have "saved" Commodore.

But it could have provided a unified interface GUI on the C64, C128, and future models possibly running on different CPUs.