A valid point.... just don't tell Heathrow Express.
I suspect that the acceptable wait time varies by time of day, because the clientele varies also. A great many business travellers come in in the early morning, and leave at close of business. Even without the 'premium' market focus, this may be UPE's best market. For these folks, time is everything. In my experience, people with a late afternoon/early evening flight get twitchy in business meetings by about 3:00 PM. Getting into the business district promptly in the morning matters greatly, too - especially if you are heading to a meeting with clients or the brass at head office. UPE with guaranteed arrival downtown is far less stressful than grabbing a limo with the uncertainty of how the roads are moving on that particular day. At these times, walk-on-and-go is good marketing.
Mid-day, I suspect there is less minute-to-minute pressure on trip times. A 20-minute headway isn't all that much more of a wait than 15 minutes (unless you have a meeting or need office time at the other end). If you assume that travellers arrive at random intervals, the average wait will be 10 mins instead of 7.5 mins. Only a quarter of passengers will face a wait that is certain to be longer than with a 15 minute headway.
Late evening is the most time sensitive. I find it's the time of day when people just want to get to their place of rest and will pay accordingly. Waits at the airport for transportation are excrutiating. There is the most anxiety about whether public transit will be safe or hassle free, especially if it doesn't drop you right at the door. (Super Shuttle into an American city after an evening flight can be cheap but hair-raising). Heathrow Express doesn't even try to capture this market....they run 30 mins after 9:30 PM. It may well be pointless to offer more than token service here in Toronto either.
I don't profess to know what the right balance, but I think it has to be considered and some experimentation is desirable. It's an easy thing to correct after a few months.
- Paul