Cream's studio music falls, at first sight, into three periods. The first ('Fresh Cream') was when the band were trying, with varying degress of success, to establish their sound. The second was the psychedelic 'Disraeli' one, when they gained their commercial success (the earlier 'Fresh Cream' material didn't sell very well). The third was the progressive material, represented by 'White Room' and so on. Then the band split, and this 'posthumous' recording was rather ignored by record buyers at the time, though a fair number of keen Cream fans bought it.
In my opinion, 'Goodbye' suggests that the band made a really big mistake in splitting when they did. There are some truly great tracks on here, easily as good as anything on their other albums, and in several cases considerably better. In some ways it represents a fourth type of Cream music, totally proficient technically, with great songwriting that had left the Blues way behind and was taking them far beyond what they'd already achieved on their earlier albums. 'Badge' is just one such example.
Personally, I think that Jack, Ginger and Eric all went downhill after this album. Blind Faith led nowhere; the Baker Gurvitz Army were interesting, but perhaps the earlier Gun were better; Jack sacrificed sheer excitement for jazz and complexity; Ginger headed into jazz, but it's a very challenging style in which few can be true masters; and Eric became very popular, but with his amplifier turned down and many fans longing for blistering 'Bluesbreaker' type material that never truly re-materialised.
Creativity often relies on tension, and there was tension a plenty in Cream, thanks to Bruce and Baker's arguments and fistfights when they were in the Graham Bond Organisation. Maybe this album is great because those tensions helped drive the creative process, and when Bruce and Baker went their separate ways, the energy was somehow lost. In some ways this album reminds me of Abbey Road; both the Beatles and Cream were supposedly finished, but both their final albums were stunningly good. How many other artists have gone out like that? None that I can think of. One can't imagine Cream going all through the Seventies banging out 'Sunshine of your love' and so on, so maybe it was time to say goodbye. But the entire heavy metal/hard rock scene derives, in large part, from Cream, even if people don't want to admit it. So in that sense, Cream might have found continued success. We'll never know. At least they never made a bad album, and this one is as good as any. If they had to call it a day, this was the best way to bow out.