From his perspective at the head of one of the few truly global commercial banks, the risks of missing out on the wave of consolidation in our mature, oversupplied and expensive banking industry must seem pretty insignificant. To understand why he is right to tread warily over here, consider the following. Start with an economy worth pounds 100bn, grow it at 2 per cent a year for 10 years and you end up with just pounds 122bn. Grow that same pounds 100bn at 7 per cent a year and you end up more or less doubling it to pounds 197bn over the same period.
The enormous power of compounding explains why HSBC's strong position in countries like China, Malaysia and India, where GDP is growing at between 5 and 10 per cent a year, is worth so much more in the long run than stakes in the economies of the US, UK and western Europe, where growth is a more pedestrian 2 to 4 per cent.
With wealth per head of population reckoned to be no more than about $720 in China, compared to $29,600 in the US, there is no reason to suppose recent levels of growth should not continue for years if not decades. If current rates continue, China will have the biggest economy in the world by the year 2020, meaning it is unlikely to be rocking the boat in Hong Kong.
The picture is actually brighter than these bald figures might suggest. In Latin America, where HSBC has acquired a leading position in the past year for about a third as much as it would have to pay for a second-liner such as Alliance & Leicester, the proportion of the population deemed to be even bankable is as low as 20 per cent. There is enormous scope for growth.
Accept that the bankable population in developing markets will grow faster than the economy as a whole, and throw in the propensity in a lot of Far Eastern markets for high levels of personal saving, and the outlook for HSBC begins to justify the seven-fold increase in its share price over the past five years. That, at any rate, is the theory. The ticklish question for Sir William is whether he can really afford to stand by and watch Midland become marginalised as the UK banking sector consolidates. Faced with the creation of another Lloyds TSB retail banking giant, HSBC's strategy could take a sharp temporary diversion.