That would require a whole bunch of grade-separated right-of-way & track that doesn't currently exist. The brilliance of the Triboro is that it's all already pretty much there.

Note also that there's a long history of truncating the G train [1].

I personally would love to see the Franklin Ave Shuttle extended to make a loop out of Brooklyn portion of the current G train route along Church Ave, connecting north-central Brooklyn to Williamsburg, Greenpoint, and Long Island City. That's probably a pipe dream.

[1] https://en.wikipedia.org/wiki/G_(New_York_City_Subway_servic...

In addition to what other people have said, the G is VERY slow.

This proposal and the G serve very different areas. The motivation is more than just "connect the three boroughs".