Here are another couple of pages from the Clyde Navigation Trust brochure referred to earlier. This is the article on "future development". Interesting if somewhat grim reading considering what History actually had up its sleeve.
The 2nd page may be a bit on the wide side. The text might not have been legible otherwise.
As I see it there are essentially 3 reasons to account for Glasgow's eventual demise as a deepsea port.
Firstly, the decline of its industrial base. This had already begun at the beginning of the 20th Century but was temporarily slowed down by the reconstruction necessary after both World Wars. The loss of Empire as well as cheaper and probably more efficient overseas competition were likely the most important factors in this process.
Secondly, the advent of containerisation. This was a development which took hold like wildfire as it dramatically reduced vessels' turnaround. Most of my working life has been spent in the shipping industry and I saw this revolution taking place over the course of a few years during the 1970's. By 1980 I reckon near enough 80% of global trade was being moved in containers. Upriver general cargo facilities like Prince's Dock and Queen's Dock, which were already seriously underutilised in the 1960's, simply became obsolete.
Thirdly, Glasgow's geographical location on the north west coast of Europe was a big disadvantage. It was therefore unable to develop as an intercontinental shipping hub like other large ports in more favourable locations, such as Felixstowe, Rotterdam and Antwerp.