You haven't disappointed me.
but I doubt anybody who backed the MSC at the time of it's construction could have foreseen that ships would grow in the way in the way they have.
They could have. Look at the Great Eastern which was decades before the canal was built. It could not sail up the canal. Steam ships were well established by the time of the canal's opening. There are a number of points about the canal. First, was why it was built in the first place when a large port is nearby. Myth in Manchester states that the Port of Liverpool overcharged. This was not the case as Liverpool always had spare capacity, so matters were negotiable. If Liverpool overcharged, Manchester organisations could always use Garston which was a separate port and if Garston was bursting at the seams because of Manchester trade, it could build extra docks. Garston had two docks by the start of the canal construction, with consideration for another. The larger Stalbridge Dock was opened in 1907, a few years after the canal opened. Manchester never extensively used Garston. In fact Elders Fyffes moved from Manchester to Garston around 1912 finding the port more convenient than Manchester Docks. The rail companies overcharged, not Liverpool.
I have read some of the legal transcripts into the hearings of the construction of the canal. Some were laughable. One was that Liverpool had little quayside storage of cargos, unlike Lancaster, and charged for lengthy storage, unlike Lancaster. The success of Liverpool was getting cargos off ships and away to the customer ASAP. The customer then could store at their location not at the port. This cleared the sheds and quays quick for the next ship giving a great turnaround and high port throughput. If you cluttered the port by not collecting your cargo then the port charged for it. Liverpool had to, to make the port efficient.
All the canal company had to do was build a railway, not a very expensive to build canal. This requires an act of parliament and of course objections from other rail operators would come in, but I would see no real objections to building their own railway adding competition.
The Rainhill trials were in the late 1820s, 60 years before the construction of the canal started, not 40. In those 60 years technology and industry advanced so quickly, the period was unrivalled to any other time in history.
You mentioned the sailing time up and down the canal and being delayed by tides as well. This was a real economic concern. And also the fact that if one of the locks is out of commission your ship may be held up for days. Indeed this did occur in the early 70s when a ship rammed a lock gate, locking up about 25 ships of all sizes in Manchester for about a week. Some lines never went back again too often.
The canal above Runcorn is now a glorified large barge canal like seen on the Continent. The container "ships" taking containers from Liverpool are motorized barges, like seen on the Continent. A few coaster sized bulk carrying ships make it to Irlam each week.
The biggest obstacles to having largish ships run up the canal are the low bridges and narrow locks. The three locks are relatedly cheaply widened, but raising the bridges is another matter. Hence why the canal will remain a glorified barge canal.
Only the section of canal that the Mersey runs through may be kept. In fact the drainage can be put back to the original river bed which parallels most of its length. The rest of the canal can be filled in above Runcorn and traffic moved to more flexible rail, which is not dependent on tides. Traffic above Runcorn is currently poor and Peel hope it will improve with their Port Salford and Port Warrington. We shall see.
The only way the canal can be used extensively above Runcorn is if Manchester turns back to manufacturing, its historical bedrock, instead of trying to be a commercial city.
So why was this expensive to construct ship canal built when logic dictated it should not be? It was built primarily to make Manchester an important city. Important cities were always ports - and that is still the case today.