Correct - designing for the web is often about compromise, and things can change very fast.
I rarely make commercial sites but I would be less concerned about how long visitors spent on the site than how many actually spent some money - and I would want to make doing that as quick and easy as possible for everyone.
The point I would stress is that it is very common for clients to assume that everyone uses the same hardware, operating system, screen resolution, browser and software that they do - but as webmasters we know that this is not the case (far from it).
It is great fun to work on a high-spec computer making bleeding edge websites that require a fast processor, the most recent operating system and some cool new third-party software, but it may not be the best way to sell stuff to the general public.
Old webmasters never die - they just degrade gracefully...